Inheritance # Chromosomes, genes and proteins Summarized Biology Notes

Learning objectives

At the end of this unit, you should be able to:
● define inheritance as the transmission of genetic information from generation to generation
● define chromosome as a thread-like structure of DNA, carrying genetic information in the form of the genes
●define the terms:
- haploid nucleus as a nucleus containing a single set of unpaired chromosomes (e.g. in gametes)
- diplod nucleus as a nucleus containing two sets of chromosomes (e.g. in body cells)
● define genes as a length of DNA coding for a specific protein
● define alleles as alternative forms of the same gene that code for different versions of the same characteristics (version of the gene)
● explain the concept of genetic code with reference to the sequence of bases in a gene to form amino acids in a correct order for a specific protein
● explain how DNA controls cell function by controlling the production of proteins, antibodies and receptors for neurotransmitters

● define the terms:

- genotype as the genetic make-up of an organism in terms of the alleles present
- phenotype as the observable features of an organism
- homozygous as having two identical alleles of a particular gene
- heterozygous as having two different alleles of a particular gene
- dominant as an allele that is expressed if it is present
- recessive as an allele that is only expressed when there is no dominant allele of the gene present
● calculate and predict the results of monohybrid crosses involving 1:1 and 3:1 ratios (use a genetic diagram)
● describe the inheritance of sex in humans with reference to XX and XY chromosomes
● describe how to use a test cross to identify an unknown genotype
● explain codominance with reference to the inheritance of ABO blood groups with the phenotypes being A, B, AB and O blood groups and the alleles I(A) , I(B)  and I (O)
● define a sex-linked characteristic as a characteristic in which the gene responsible is located on a sex chromosome predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios (use a genetic diagram)

Chromosomes, genes and proteins

In an earlier post titled "Biological Molecules"  , you learnt that every cell has a nucleus. Inheritance is the transmission of genetic information from generation to generation. This is done through the transfer of chromosomes, which are tound in the nucleus of the cell. Chromosomes are thread-like structures of DNA, carrying genetic information in the form of genes. Chromosomes carry the instructions from one generation to the new generation, which enable it to resemble the parent generation.

DNA

Chromosomes are made of deoxyribonudeic acid, known as DNA, and protein. The DNA in a cell carries the instructions for the production of protein in a cell. A DNA molecule is made up of subunits called nucleotides. Each nucleotide is made up of three chemical groups:
● a sugar molecule, called deoxyribose
● a base: there are four of these called adenine (A), thymine (T), guanine (G) and qytosine (C), I  remember them better by memorizing this word (Can Go To America)

● a phosphate group represented by the letter P.

- Every living cell contains DNA with the exception of the red blood cell. Prokaryotic cells may contain one DNA molecule, whereas eukaryotic cells contain several DNA molecules. The sequence of bases in the DNA determines the sequence of amino acids in the protein. Humans have 46 DNA molecules in their cells when they are not dividing, because each of the 46 chromosomes contains one DNA molecule. In humans, life begins as a single cell. In this case, a sperm cell contains 23 DNA molecules from the father and an egg cell contains 23 DNA molecules from the mother. I his is Vital for the proper development of a single cell into a complete and normal adult organism. You can refer to our earlier post titled "cell division". These instructions tell the cell what proteins to make. DNA is defined as the hereditay material contained in the chromosomes. Each chromosome is one very long molecule of DNA. The DNA molecule may contain instructions for many different proteins. A section of a DNA molecule that gives instructions for making any one kind of protein is called a gene. Each chromosome contains many genes. A gene is defined as a length of DNA cOding for a specific protein. Figure below shows that the sequence of bases in the DNA determines the sequence of amino acids in the protein.

- It is the sequence of bases in the DNA molecule that decides which amino acids are used and in which order they are joined. Each group of three bases stands for one amino acid, for example, the triplet of bases adenine-thymine-guanine (ATG) specifies the amino acid methionine, the base triplet thymine-cytosine-thymine (TCT) specifies the amino acid serine and the triplet GGT stands for glycine. The tripeptide methionine-serine-glycine would be specified by the DNA code ATG-TCT-GGT. Amino acids are encoded by three nucleotides. The DNA base sequence determines the sequence of amino acids in a peptide. Look at figure above.

The structure of DNA

DNA is a substance that belongs to a group of chemicals called nucleic acids. A nucleic acid is a long molecule found in, or associated with, the nucleus of cells.  There are two forms of nucleic acid:
● DNA - deoxyribonucleic acid
● RNA ribonucleic acid.

For the past hundred years, scientists have been studying cells and looking for ways in which information is passed on to offspring. We now know that the nuclei of cells contain thread-like structures of DNA, bound by protein. These structures are called chromosomes. Chromosomes' numbers are always even. The reason is that they exist in pairs. Ihe chromosomes belonging toa pair are called homologous chromosomes because they are similar in structure. One chromosome of each homologous pair was innerited from one parent and the second from the other parent. For example, of the 46 chromosomes, 23 came originally from each parent. Each chromosome has two chromatids, so there are four chromatids in each pair of homologous chromosomes. You can see this from the photomicrographs in Figure below . Here, human chromosomes are shown at the stage of nuclear division, and also cut out from a photograph and arranged in descending order of size. Homologous pairs are traditionally numbered in this way so as to identify them

 Diploid and haploid nuclei

Each cell in your body has 46 chromosomes. If you were able to look at the chromosomes in the nucleus of your cells, you would see 23 pairs of chromosomes. Each single chromosome of a pair is called a homologous chromosome. Homologous chromosomes are chromosome pairs, identical in length, gene position, centromere location, shape and appearance. The position of the genes on each homologous chromosome is the same, however, the genes may contain different alleles. You inherited 23 of these chromosomes from your father and 23 from your mother. The 23 chromosomes in the nucleus of the sperm joined with the 23 chromosomes in the ovum at the moment of fertilisation. The two sets of chromosomes then appear in the fertilised ovum as 23 pairs of homologous chromosomes.

In our previous post,  you learnt that a chromosome is one very long molecule of DNA, which is made up of a double strand of nucleic acid. A pair of homologous chromosomes therefore consists of two chromosomes, each of which is a molecule of DNA. The DNA molecule is a series of codes that carry instructions to make polypeptide molecules. A DNA molecule may carry, say, 100 separate codes for the production of 100 different polypeptide molecules. Each code occupies a particular place on the chromosome called a locus. This molecule of DNA will have 100 loci. The name given to each locus on a chromosome is a gene. A gene is the functional unit of a chromosome. It is the region of a chromosome that carries the genetic code to make a particular polypeptide molecule. Gamete cells containing one set of 23 chromosomes are known as haploid cells. A haploid nucleus is defined as a nucleus containing a single set of unpaired chromosomes (for example, in gametes) They have only one chromosome of each homologous pair. This means they have only one locus of any particular gene and carry only one copy of the gene They are represented by the symbol n. All other cells in an organism have two sets of chromosomes. They are called diploid cells and they contain diploid nuclei. A diploid nucleus is a nucleus Containing two sets of chromosomes (for example, in body cells). In humans, the body cells contain 46 chromosomes. Diploid cells have pairs of homologous chromosomes. This means they have two loci and carry two copies of each gene. These cells and nuclei are represented by the symbol 2n. Figure below shows how one diploid cell forms two haploid cells by meiosis. Before studying the subject of genetics and seeing how we can predict the way in which inheritance of characteristics takes place, we need to go just one step further on the topic of genes.

Genes

Chromosomes contain genes. A gene is defined as alength of DNA coding for a specific
protein. Characteristics are transmitted from parents to offspring via the genes. Genes control important things, such as how well your organs work. They also control things that are not so important, such as the colour of your hair and the shape of your face. Each of your cells, except your sex cells or gametes, containsa pair of each kind of gene. All the cells of an organism have the same pairs of genes. One gene of each pair comes from the mother and the other gene comes from the father. The pairs of genes are in the same place on the chromosomes.

Genes and fertilisation

The ability to roll your tongue is inherited. If a tongue-rolling man who Is neterozygous. Rr, marries a non-tongue-rolling woman with the  genotype rr, will their children be able to roll their tongues or not? The ova in the woman's ovaries are also made by melosIs. As she is homozygous, rr, all of the ova will carry an r gene. During sexual intercourse, hundreds of thousands of sperm will begin a journey towards the ovum. About half of the sperm will carry an R gene, and half will carry an r gene. If there is an ovum in the woman's oviduct, it will probably be fertilised. There is an equal chance of either kind of sperm getting there first:
● lf a sperm carrying an R gene fertilises the ovum, then the zygote will have an R gene from its father and an r gene from its mother. The baby will have the genotype Rr, like its father, and be a tongue roller.
● if a sperm carrying an r gene fertilises the ovum, then the zygote will have an r gene from each parent. The baby will have the genotype rr, like its mother, and be a non-tongue roller.

Alleles

Each chromosome of a homologous pair carries genes for the same characteristic in the same place. Alleles are alternative forms of the same gene. Alleles are represented by letters. The alleles must have the same letter, but the dominant allele is always in capitals. For example, the two chromosomes belonging to pair number 1 might each carry a gene for tongue rolling. In humans, there are two kinds of tongue-rolling genes. One kind, R, allows you to roll your tongue. The other kind, r, does not. The tongue-rolling allele will be at exactly the same position, or locus, on each chromosome. Figure below shows how homologous  chromosomes have the genes for the same length and carry the same gene sequences.

 Inheritance of tongue rolling

Let us look at the pair of alleles, which control tongue rolling, to see how they behave and how tongue rolling is inherited. As shown in Figure below, there are three possible combinations of alleles for tongue rolling. You might have:
● two R genes, RR
● two r genes, rr
● one of each, Rr.

If the two alleles for tongue rolling are the same, that is, RR or rr, then you are said to be homozygous for tongue rolling. If the two alleles are different, that is, Rr, then you are heterozygous for tongue rolling.

Monohybrid inheritance
Genotype and phenotype

The genes that you have are described as your genotype. As we have seen, for tonque rolling, there are three possible genotypes: RR, Rr or r. The genotype determines whether or not you can roll your tongue. The effect that the genotype has is called your phenotype. Your phenotype for tongue rolling is either being able to roll your tongue, or not being able to do it. The phenotype can be seen, whereas the genotype cannot be seen. The phenotype is the outward appearance of an individual.

Dominant and recessive genes

There are three kinds of genotype for tongue rolling, but only two kinds of phenotype. How does this occur? It happens because the tongue rolling gene, R, is dominant over tne non-tongue-rolling gene, r. f you are heterozygous for tongue rolling, Rr, then it is only the R gene that actually has any effect on the phenotype. You can roll your tongue. The effect of the r gene is hidden by the R one. The r gene is said to be a recessive gene. This is summarised as follows in Table below:

Homozygous and heterozygouss

Homozygous describes a genotype consisting of two identical alleles of a particular gene ata given locus, while heterozygous describes a genotype consisting of two different alleles of a particular gene at a locus.

Monohybrid crosses

There are two types of genetic crosses, monohybrid and dihybrid, but we will focus on monohybrid. Monohybrid is the passing on of a single characteristic, for example, tongue rolling is a single characteristic that is passed on from the parents to oftspring. There is a standard way of writing a monohybrid cross:
● First, write down the phenotypes and genotypes of the parents.
● Next, write down the different types of gametes they can make:

● The next step is to write down what might happen during fertilisation. Either kind of sperm might fuse with an ovum. A table can be used to show the fusion of each of the two kinds of sperm with the ovum:

(Half of the offspring are heterozygous tongue rollers, and half are homozygous non-tongue rollers. There are equal numbers of offspring who are tongue rollers and non-tongue rollers. It is usual to express the results of a genetic cross in terms of a ratio. The probability of being a tongue roller or non-tongue roller is in the ratio 1:1. A 1:1 ratio in offspring usually indicates a cross between a heterozygous parent and a homozygous recessive parent. In this example, Rr and r. A 3:1 ratio in offspring indicates a cross between two heterozygous organisms.)

Solving genetics problems

In an examination, you may be given a particular genetics problem to work out. There is a standard procedure to follow in working out the answer ana presenting it, so that others can see your calculations.
1. Read the information through twice very carefully.
2. DeciIde whether or not one gene is dominant over the other.
3. Decide on the symbols to use. Usually you should use the capital letter of the first letter of the dominant characteristic. The small version of this letter is used for the recessive characteristic. For example, if red is dominant and yellow is recessive, you can write:
Let R = red, let r = yellow.
4. Write out in words the parental phenotypes.
5. Write out in symbols the parental genotypes.
6. Write out the contents of the gametes
7. Write out the F, offspring genotypes.
8. Write out the F, offspring phenotypes
9. If the phenotypes are crossed, continue writing out the genetic diagram from Step 5 again. This time these will be the F, offspring phenotypes, and so on.

In another example, the gene coding for normal skin colour pigments is represented by the capital letter (H) and the recessive gene for albinism is represented by the small letter (h). In a couple, if both parents' cells contain heterozygous genes, what is the likelihood of having albino children?
● Parental phenotypes: male x female
● Parental genotypes: Hh Hh
● Gametes: (H) and (h) × (H) and (h)
● Offspring phenotypes: HH = homozygous dominant; normal skin pigment
● Hh = heterozygous; normal skin colour pigment
● Hh = heterozygous; normal skin colour pigment
● hh = heterozygous recessive; albino
● Ratio = 3:1
● Percentage = 75% normal skin colour and 25% albino
● Offspring genotypes

Sex inheritance in humans

Sex is determined by the sex chromosomes X and Y. The X and Y chromosomes differ in size; the X chromosome is much longer than the Y chromosome.
● A woman's chromosomes are both alike and are called X chromosomes. She has the genotype XX.
● A man has only one X chromosome. The other, smaller one is a Y chromosome. He has the genotype XY.
Figure below shows the sex chromosomes in a human male. You can work out sex inheritance in the same way as for any other characteristic, but using the letter symbols to describe whole chromosomes, rather than individual genes.

 Test cross

It is often possible to decide on the genotype of an organism from the phenotype. If the phenotype shows the recessive gene, it must be homozygous recessive, say, tt. If the organism shows the dominant phenotype, it could be either:
TT                     or         Tt
homozygous             heterozygous

The way to determine the phenotype in this situation is to cross the unknown organism with an organism that is homozygous recessive. This is called a test cross.
● If the offspring all show the dominant characteristic, the organism must have been homozygous.
● If the offspring show a 1:1 ratio, the organism must have been heterozygous.

Codominance and inheritance of blood groups
Sometimes, instead of one allele being dominant to the other, both alleles are equally dominant and their effect can be seen in the heterozygous organism. This is called codominance.

Inheritance of blood groups
There are four human blood groups: Group A, Group B, Group AB and Group O. These blood groups are controlled by a single gene, which is normally represented by the letter. This gene has three alleles that are represented by the superscript letters A for blood group A, B for blood group B and o for blood group O.
● Alleles A and B are both dominant to allele O.
● Alleles A and B are codominant.
The four blood group phenotypes have the following genotypes:

Phenotype                  Genotype
blood group A             I (A) I(O), I(A)I(A)  
blood group B             I(B)I(O), I(B)I(B)
blood group AB           I(AB)
blood group O             I(O)I(O)

Sex linkage

Sex-linked is a characteristic in which the gene responsible for a condition is located on a sex chromosome. Haemophilla and colour blindness are both medical conditions caused by sex-linked genes:
● In haemophilia, blood clotting is defective and the sufferer might bleed uncontrollably from the slightest cut.
● Sex-linked colour blindness is usually the inability to distinguish the colour red from green.

In both these conditions, the defective gene is found on the X chromosome. These conditions are termed sex-linked. The male, XY, suffers the effect of the condition. The female, XX, usually does not suffer from the condition, but she may carry the defective gene. Females have twO X chromosomes. The presence of a recessive allele on one of the X Chromosomes is usually cancelled out, when expressing the gene in the phenotype, by the dominant allele on the other X chromosome. In males, the Y chromosome is relatively small and does not carry many genes. The presence of a recessive allele on a region of an X chromosome, which does not have a corresponding region on the Y chromosome, will result in the appearance of the recessive trait in the phenotype. This is why sex-linked recessive conditions appear only in males and not in females Haemophilia and colour blindness are both conditions resulting from recessive allele situated on the X chromosome. Haemophilla is a sex-linked recessive condition that prevents the formation of a substance that increases the rate of blood clotting. The gene controlling the condition s situated on the X chromosome, as shown in Figure below The gene appears in two forms normal  (dominant) and haemophilia (recessive).

Summary of terms used in genetics

Summary

● DNA is a hereditary material contained in the chromosomes.
● Chromosomes are thread-like structures containing DNA and are found in the nucleus of every cell.
● Homologous chromosomes are identical in length, shape and appearance, and have genes of the same characteristic In the same place.
● Haploid nuclei contain one set of chromosomes.
● Diploid nuclei contain two sets oft chromosomes.
● A gene is a length of DNA coding for a specific protein.
● Alleles are alternative forms of the same gene, which code for different versions of the same characteristics.
● Inheritance is the transmission of genetic information from generation to generation; leads to continuity of, and variation within, a species.
● The genotype is a genetic constitution of the organism, which is invisible.
● The phenotype is the observable characteristics or appearance of the organism.
● The  genotype is termed homozygous when both alleles are identical genotype
● The genotype is termed heterozygous when the alleles are different.
● Dominant alleles have the same etfect on the organism, whether homozygous or heterozygous.
● Recessive alleles express themselves only when homozygous
● Sex is inherited in humans by the sex chromosomes, XX in females and XY in males. when two alleles exist, but neither is dominant nor recessive, they are called codominant, for example, inheritance of blood groups in humans.

 End due to Long paragraph see next post about  Variation. Posted by Mr Steven Artssen.