Biology: Inheritance

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Genetic Terms

A gene is the physical unit of heredity. It is a sequence of DNA that occupies a specific site, or locus, on a chromosome, and codes for a polypeptide. Polypeptides and proteins determine structures and functions in living organisms, and so the gene is also thought of as the sequence of DNA that codes for a characteristic.

There are several genes that control coat colour in dogs. In one, the nucleotide sequence codes for an enzyme that makes the fur black, but a small difference in the sequence gives a slightly altered pigment molecule that makes the fur brown. Small changes in nucleotide sequences of the same gene make small changes in the polypeptide they encode and produced different characteristics. The different sequences are called alleles. A single gene may have one, two or many alleles. Alleles always occupy the same locus because they are versions of the same gene.

A gene controls a characteristic and alleles are different versions of the gene in the same way that a car (gene) could be a BMW or a Porsche (alleles).

Organism Gene Alleles

Human Number of freckles 5; 6

Freckles Present; absent

Rhesus blood group Positive; negative

Sweet Pea Height Tall; dwarf

Seed colour Yellow; green

Flower position Terminal; Axial

A diploid individual has one of each chromosome from each parent, and , therefore, one copy of each gene from each parent. If the alleles of a particular gene are the same from both parents, the individual is homozygous for that gene. If they are different, the individual is heterozygous for that gene.

The genotype of an individual is all the alleles that they contain. Their phenotype can be described as their appearance, but it is much more than that, because it includes characteristics that cannot be seen, such as blood group. Both the genotype and the environment determine the characteristics of an individual so the phenotype is better thought of as the expression of the genotype in a specific environment.

There is a gene that codes for stem height in sweet pea and some other plants. An allele of this gene, given the symbol T, codes for the production of gibberellin, a growth regulator that elongates the stem, so plants with the T allele are tall. An alternative allele, given the symbol t, does not code for the production of gibberellin so the plants that do not have a T allele are dwarf. Each plant has two alleles, which gives three possible combinations.

Allele combination Effect Phenotype Description of genotype

tt No gibberellin produced Dwarf Homozygous recessive

TT Gibberellin produced Tall Homozygous dominant

Tt Gibberellin produced Tall Heterozygous

The T allele is always expressed when it is present, so it is described as dominant. The t allele is not expressed in the presence of the T allele, and is only expressed when it is homozygous. It is described as recessive.

For a heterozygous gene, the dominant allele is always written before the recessive allele e.g. Tt.

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Monohybrid Inheritance

Monohybrid inheritance is the inheritance of a single gene, such as that controlling plant height or seed colour.

Gregor Mendel's classic experiments investigated inheritance in plants of the garden pea, Pisum sativum. Because of their agricultural importance there were several varieties available that were true breeding, although nobody understood why. Peas were a useful choice for experiments because:

1. They are easy to grow.

2. Their flowers can self-fertilise and cross-fertilise.

3. They make flowers and fruit in the same year.

4. They make a large number of seeds from each cross. This means that when the phenotypes of the next generation are counted, their numbers make them statistically meaningful.

To study how characteristics were inherited, Mendel chose pairs of contrasting characteristics, such as tall or dwarf plants, plants with round or wrinkled seeds and plants with yellow or green seeds. He was fortunate, or perhaps skilled, in his choice because these characteristics are:

1. Controlled by single genes.

2. Controlled by genes on different chromosomes.

3. Clear-cut and easy to tell apart.

These characteristics are examples of discontinuous variation. However, most characteristics show continuous variation. They have a range of values and are controlled by a number of genes, for instance height, in humans.

Peas have been used in crop rotation in Europe since the 8th century, to increase nitrate levels in soil. By the time of Mendel, in the 19th century, they were very important in the 4-year crop rotation.

Genetic Diagrams

A diagram of genetic cross shows:

1. The generations, i.e. parents, first generation (F1), second generation (F2), and, sometimes, further generations.

2. The genotypes of parents and offspring.

3. The phenotypes of parents and offspring.

4. The alleles present in the gametes.

5. The symbols for the alleles are defined.

F1 stands for the first filial generation. It is the offspring of the parents of the cross. F2 stands for the second filial generation. It is the offspring of a self-fertilised F1 plant or of a cross between members of the F1 generation. It is also the grandchildren of the original parents. Filial is a word that means "relating to a son or daughter".

Instructions for Writing a Diagram for a Genetic Cross

1. Choose suitable symbols for the alleles.

a. Use a single letter for each characteristic, for example, the first letter of one of the contrasting features.

b. If possible, use a letter in which the upper and lower cases differ in shape and size.

c. Use the upper case for the dominant characteristic and the lower case letter for the recessive.

2. Write the parents' genotypes with the appropriate pairs of letters. Label "Genotype of Parents" and state their phenotypes.

3. Show the gametes produced by each parent. Circle and label them "Gametes".

4. Use a matrix, called a Punnett Square, to show the results of the possible combinations that result from random crossing of all the gametes. Label "Genotype of F1".

5. Show the phenotype of each F1 genotype in the Punnett Square.

6. Indicate the ratio of the phenotypes.

This diagram shows the inheritance of the gene for plant height:

1. The gene has two alleles: T represents the allele for tall and t represents the allele for dwarf.

2. As the parents are homozygous, each only makes one type of gamete, so only one is shown.

3. In the F1, all offspring are heterozygous and their phenotype is tall.

4. F1 individuals each make the two types of gamete.

5. In the F2, 3/4 of the offspring are tall and 1/4 are dwarf.

In a monohybrid cross, two heterozygous individuals produce offspring with a phenotypic ratio of 3 with the dominant characteristic: 1 with the recessive characteristic.

Test Cross or Back Cross

If an organism has a dominant characteristic, it could be homozygous dominant or heterozygous. The test cross shows if a dominant characteristic is determined by one or two dominant alleles.

A tall pea plant, for example, could be pure-breeding, TT, but it could also be Tt. It is not possible to tell from the appearance. To test its genotype, the tall plant is crossed with a dwarf plant. The dwarf plants phenotype has only one possible genotype: it is homozygous recessive, tt. If the tall plant were TT, all the F1 would be tall, but if it were Tt, 50% would be tall and 50% would be dwarf.

A back cross is so called because it is as if the test organism were being crossed with the homozygous recessive parent of the original cross that produced it.

In a back cross, the genotype being tested is crossed with a homozygous recessive individual. If the F1 all have the dominant character, the parent was homozygous dominant; if 50% have, the parent was heterozygous.

When Neither Allele is Dominant

1. Co-dominance

When genes are co-dominant, both alleles in a heterozygote are expressed individually. As a result, the heterozygote has a combination of both of the homozygotes' characteristics, e.g. in the human ABO blood group system, the I gene has three alleles IA, IB and IO. Homozygous parents who have the genotype IBIB have the B antigen on their red blood cells and are blood group B. Homozygous parents who have the genotype IAIA have the A antigen on their red blood cells and are blood group A. Their offspring will have the genotype IAIB. Both alleles are expressed: they have both A and B antigens on their red blood cells so they are blood group AB. This is an example of co-dominance.

Similarly, chickens with black feathers and white feathers can produce speckled offspring as neither the allele for black feathers nor the allele for white feathers is dominant.

In co-dominance, both alleles of a gene are expressed. The heterozygote's phenotype shows the phenotypes of both homozygotes. In incomplete dominance, the heterozygote phenotype is intermediate between that of the two homozygotes.

2. Incomplete Dominance

For some genes, the phenotype of the heterozygote is intermediate between the two parental genotypes, rather than their both being expressed, e.g. red flowered carnations crossed with white-flowered carnations have an F1 with pink flowers. This is described as incomplete dominance. Neither the allele for red petals nor white petals is completely dominant and so the allele symbols are not given an upper or lower case letter. Instead, they have the symbols R or CR for red and W or CW for white.

In a cross between carnations with red and white flowers, all flowers of the F1 generation are pink. They produce two different types of gamete so when the interbreed, the ratio of colours in the F2 generation is 1 red: 2 pink: 1 white.

As a result of carrying out his experiments, especially those that produced the 1:2:1 ratio, Mendel formulated his first "law of inheritance", the law of segregation, which states: The characteristics of an organism are determined by factors (alleles) which occur in pairs. Only one of a pair is present in each gamete.

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Independent Assortment

The diagrams show a cell at metaphase I of meiosis, with the maternal chromosomes coloured black and the paternal chromosomes coloured red. In relation to genes A/a and B/b, the mother's phenotype is AB and the father's is ab. In diagram 1, the homologous chromosomes are arranged so that the alleles A and B got to one pole and a and b got to the other, so the gametes produced are written as AB and ab.

In diagram 2, the orientation of the shorter pair of chromosomes is reversed, so that the alleles A and b go to one pole and a and B got to the other. The gametes produced are written as Ab and aB.

In this way, the mechanism of meiosis explains the observation that either of a pair of alleles can combine with either or another pair of alleles. The maternal and paternal chromosomes, and therefore the alleles they contain, mix with each other in many combinations in the gametes. This is independent assortment of chromosomes. The genes A and B behave independently in relation to each other and so they are described as being unlinked. A is equally likely to combine in a gamete with B or b. It accounts for the genetic ratios observed when the simultaneous inheritance of two genes is investigated in dihybrid inheritance.

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Dihybrid Inheritance

Dihybrid inheritance is the simultaneous inheritance of two unlinked gene, i.e. genes on different chromosomes. Mendel knew from his early experiments with monohybrid crosses that, for pea seeds, yellow colour was dominant to green and that round seed shape was dominant to wrinkled. He crossed plants with two pairs of contrasting characters:

1. The allele for round seeds is R and the allele for wrinkled seeds is r.

2. The allele for yellow seeds is Y and the allele for green seeds is y.

Plants with the genotype RRYY are homozygous for the two dominant characters, round and yellow seeds. They were crossed with plants with the genotype rryy, homozygous for the two recessive characters, wrinkled and green seeds. Each parent produced only one type of gamete, RY or ry. All the F1 plants had the genotype RrYy, and had the dominant characteristics, e.g. for round, yellow seeds.

Each gamete of the F1 plants contained an allele for shape and an allele for colour. In making the gametes, the R allele would combine with either the Y or y allele with equal probability, and therefore, in equal proportions. The Y allele would combine with either the R or r allele in equal proportions. Each parent, therefore, produced four types of gamete in equal proportions: RY, Ry, rY, ry.

When the F1 plants self-fertilised, each gamete type in the pollen would combine with all four gamete types in the oospheres, with equal probability, and therefore, in equal proportions. When the F1 were self-fertilised, they produced seeds that were the F2 generation. The F2 seeds were of four different combinations of shape and colour, as shown in the Punnett Square.

In a dihybrid cross, both genes must be represented in the gamete, so they are written with two symbols, i.e. one allele for each gene.

In a dihybrid cross, two individuals heterozygous at both genes produce offspring with a phenotypic ratio of 9:3:3:1.

The Punnett square shows that if the colour gene alone were considered, the ratio 12 yellow: 4 green, i.e. 3:1 would be obtained. Similarly, if the texture gene were considered, the ratio 12 smooth : 4 wrinkled, i.e. 3:1 would be obtained. In each case, the monohybrid ratio is obtained, because only one gene is being considered at a time.

To calculate the ratio of progeny, the total number is divided by the number of homozygous recessive individuals. In a cross, it is most likely that the ratio of progeny will be exactly as predicted. But if the ratio approximates to 9:3:3:1, it can be assumed that the progeny are derived from a dihybrid cross.

The formulation of the dihybrid ratio led to the "second law of inheritance", also attributed to Mendel. It states: Either one of a pair of contrasted characters may combine with either of another pair. With our current understanding of genetics this statement can be written as: Each member of a pair of alleles may combine randomly with either of another pair on a different chromosome.

The Dihybrid Test Cross

A pea plant with round, yellow seeds could have any of the following genotypes: RRYY; RrYY; RRYy; RrYy. The monohybrid test cross tests a genotype by crossing it with an individual that is homozygous recessive, e.g. tt. The dihybrid test cross tests a genotype by crossing it with an individual that is homozygous recessive for both genes, i.e. rryy. The ratios of phenotypes in the progeny indicate the genotype of the parent. In each cross, the parental phenotypes are round, yellow and wrinkled, green.

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Linkage

The alleles of two genes that are on the same chromosome cannot segregate independently, i.e. they cannot move to opposite poles of the cell at meiosis. This is because they are on the same physical structure - the chromosome - so they must move together. They are described as linked. The cell in the diagram makes gametes DE and de:

In some cells undergoing meiosis, genetic crossing over would occur between genes D/d and E/e, producing the 4 gamete types DE, De, dE and de. DE and de are the "parental" genotypes and De and dE are the "recombinant" genotypes.

The combination of characteristics is described as parental when traits associated with D and E or with d and e occur together. The phenotypes are described as recombinant when traits associated with D and e or with d and E occur together.

Crossing over is a rare event and does not happen in most cells, so the majority of gametes would still be parental, DE and de. Thus the number of gametes with the different genotypes is not equal, so Mendelian ratios are not produced in the next generation and the majority of phenotypes in the offspring would have the parental phenotypes. The further apart two genes are on a chromosome, the more opportunity there is for a crossover to occur between them. This leads to more recombinant gametes and therefore more offspring with recombinant phenotypes.

Recognising Linkage

if the numbers of progeny with different combinations of characteristics do not correspond with Mendelian ratios, it is likely that genes are linked. This is a common explanation for rejecting a null hypothesis in a genetics experiment.

In an experiment with Drosophila, the fruit fly, crossing flies that have red eyes and yellow bodies with flies that have brown eyes and dark bodies produced an F2 generation with:

Red eyes, yellow body (parental) = 126

Brown eyes, dark body (parental) = 39

Red eyes, dark body (recombinant) = 5

Brown eyes, yellow body (recombinant) = 3

There are four phenotype classes, as expected but not in a 9:3:3:1