Chapter 14

Gregor Mendel performed experiments on inheritance that changed our understanding of the process in several ways. His deductions were made in the 1860s in the absence of knowledge concerning chromosomes and DNA. One of his most significant conclusions was that heredity was particulate not a blending process.

Mendel studied heredity in garden peas (Fig. 14.2). These peas were good subjects because they sexually reproduced in ways that the paternity of the offspring could be controlled. One key was that Mendel considered the inheritance of each trait separately. He was also fortunate in the traits he chose to study (i.e., seed shape, seed color, seed coat color, pod shape, pod color, flower position, and stem length). (Fig. 14.1)

Mendel's Law of Segregation:

1) There are alternative forms for genes, the units that determine heritable characteristics (these are called alleles).

2) For each inherited characteristic, an individual has two alleles (one from each parent).

3) A gamete carries only one allele for each characteristic because alleles separate (segregate) during gamete production.

4) When two alleles are different, one is fully expressed (dominant) and the other is completely masked (recessive).

Law of Segregation: allele pairs separate during gamete formation and a paired condition is restored by the random fusion of gametes at fertilization.

When controlled matings are made the first individuals mated are referred to as the P (parental) generation. Their offspring are the first filial (F₁) generation. The offspring of F₁XF₁ are the F₂ (second filial) generation.

Dominant alleles are symbolized with capital letters, recessive alleles by lower case letters. (Fig. 14.6)

AAà homozygous dominant

Aaà heterozygous

aaà homozygous recessive

AA and Aa have the same expression (phenotype) for this trait, yet have different underlying genetic blueprints (genotypes).

A testcross is a cross between an individual and another who is homozygous recessive for the trait(s) being studied. (Fig. 14.7)

There are two laws of probability useful for understanding genetics.

1) The probability of the simultaneous occurrence of two independent events is the product of their individual probabilities. (Fig. 14.9)

2) The probability of any of a set of mutually exclusive events occurring is the sum of their individual probabilities. (Fig. 14.9)

An example of the independence rule with dice is the probability of throwing snake eyes (1,1). 1/6 X 1/6 = 1/36

An example of the mutually exclusive rule with dice is the probability of throwing an odd number on a die (1,3,5). 1/6 + 1/6 + 1/6 = 3/6 = ½

In genetics, the probability of a heterozygous parent (Aa) including a recessive allele (a) in any gamete is ½. The probability of two heterozygous parents each contributing an (a) to the gametes that will fuse to form the zygote is ½ X ½ = ¼. This is from the independence rule.

The probability that Aa X Aa produce a heterozygous offspring can occur in either of two mutually exclusive ways: the male can contribute an (A) and the female an (a) or vice versa. Each probability is ¼ so the probability that either occurs is ¼ + ¼ = ½

So, if

P: Aa X Aa

yields

F₁ Aa

And

F₁ X F₁

yields

F₂ ¼ AA: 2/4Aa: 1/4aa

These results are significant. They are for a monohybrid cross. (Fig. 14.6)

F₂ genotypic ratio 1:2:1

F₂ phenotypic ratio 3:1

When Mendel studied the inheritance of two traits simultaneously the proportions of offspring phenotypes seemed to be predictable based on the product law of independent events and the summation law of mutually exclusive events.

Mendel proposed the Law of Independent Assortment which states that each allele pair segregates independently from any other allele pair during gamete formation.

In a dihybrid cross:

P: AABB X aabb

Yields

F1: AaBb

And F1 X F1

Yields a phenotypic ratio for

F2: 9A_B_: 3A_bb: 3aaB_: 1aabb

The F2 genotypic ratio would be 1:2:1:2:4:2:1:2:1 (Fig. 14.5)

The phenotypic ratios that Mendel found often are found in monohybrid or dihybrid experimental crosses. However, sometimes different ratios are found. These unusual ratios indicate that there are confounding factors that influence heredity and that Mendel had not understood these confounding factors when he generated his conclusions. Modern scientists recognize that certain unusual phenotypic ratios indicate specific modes of inheritance that Mendel didn't appreciate. We will now consider some of these modes of inheritance.

One allele may not be completely dominant over another.

In incomplete dominance heterozygous individuals have phenotypes intermediate between the homozygous dominant and recessive phenotypes. This is usually due to the phenotype being dependent on the total amount of some chemical, and that chemical's concentration being dependent on enzymes produced under the instructions provided by the dominant allele. Individuals that are AA may produce more of the enzyme than individuals that are Aa. Phenotypic and genotypic ratios are identical for traits exhibiting incomplete dominance. Note that in incomplete dominance the heterozygote essentially expresses the dominant phenotype, but to a lesser degree. For example, the heterozygote may have pink flowers rather than red flowers. (Fig. 14.10)

Codominance is similar to incomplete dominance in that the phenotypic and genotypic ratios are identical. But, in codominance the heterozygous individual expresses the phenotypes of both homozygous genotypes. An example of codominance is the ABO blood antigen system in humans.

The ABO blood antigen system also demonstrates that more than two alleles can exist in a population for any particular gene. Any single diploid individual can have at maximum two alleles, but more may exist in the population. (Fig. 14.11)

Sometimes one gene may control more than one trait. This is called pleiotropy.

An individual's phenotype is the combined result of both its genotype and the phenotype it experiences. Penetrance is the proportion of the individuals that show the phenotype expected based on their genotype (this refers to those genotypes controlling an unusual phenotype). Expressivity is the degree to which a particular gene is expressed in individuals showing that trait.

Epistasis is where one gene interferes with the expression of another gene (Fig. 14.12). For example:

AàBàC

Where one gene codes for the enzyme controlling the first reaction and a different gene controls the second reaction.

The additive effect of two or more genes on a single phenotypic character producing that show apparently continuous variation between phenotypes is referred to as polygenic inheritance.

For example, dark skin is influenced by three genes that may have contributing alleles A, B and C. An individual AABBCC would have very dark skin and an individual aabbcc would have very light skin. (Fig. 14.13)