Mendelian Inheritance

Outcomes:

• • Explain the Mendelian model of inheritance

  • Use pedigrees to analyse patterns of inheritance.

  • Discuss the importance of the environment on the phenotype

  • Extension: Explain sex-linked inheritance and identify variation from the basic Mendelian ratios.

Determining chances of inheritance

Gregor Mendel

Gregor Mendel is known as the father of genetics. He was an Austrian monk who carried out experiments in his garden using pea plants and discovered some of the fundamental laws of heredity we still rely on today.

Mendel looked at the features of pea plants, tall or short (dwarf). He assigned letters to represent these characteristics (known as alleles): T = tall and t = dwarf. 

Mendel bred the plants observing the height in the next generation. He discovered that tall pea plants could be either pure breeding (TT or homozygous dominant) or hybrid (Tt or heterozygous). The dwarf plants would be tt (homozygous recessive). 

Mendel also used the following notation to represent different levels of generations:

• P = parent generation

• F1 = 1st filial generation

• F2 = 2nd filial generation.

An easy way to predict the outcome of crossbreeding plants with certain traits is to use a Punnett square, a specialised grid invented by Reginald Punnett. To use a Punnett square, you need to know the genotype of the parents and then work out what genetic information they could pass on to their offspring via the gametes. 

For example, we know a heterozygous, tall pea plant has the genotype Tt. This means that when the plant makes its gametes, they could contain either a T or a t, as gametes only carry one of the alleles from the parent. In contrast, a homozygous tall pea plant has the genotype TT, so its gametes will all contain a T.

Mendel's Laws of Heredity

Mendel concluded that biological inheritance is determined by factors that are passed from one generation to the next. These factors that determine traits are called genes.

Alleles- different forms of genes, such as genes for height can either produce genes for tall plants or genes for short plants.

Law of Segregation (1st Law)- Every organism has two alleles of each gene and when gametes are produced the alleles separate.

Phenotype- the way an organism looks and behaves (tall or short)

Genotype- the allele combination of an organism (TT, tt, Tt)

An organism is homozygous for a trait if the two alleles for the trait is the same. (TT- homozygous dominant, tt- homozygous recessive)

An organism is heterozygous for a trait if the two alleles for the trait are different. (Tt- heterozygous)

Law of Independent Assortment (2nd Law)- genes for different traits are inherited independently of each other..

In a dihybrid cross (two traits) you can see both of Mendels laws at work.

The genotype provides the basic instructions for physical characteristics through genes that control specific traits such as skin colour or body shape. The environment can only modify the final appearance, and the effect is not usually permanent. The ability to respond to the environment is also controlled genetically.

Questions:

Explain how the following examples demonstrate the influence of the environment on phenotype. Which particular aspect of the environment is affecting a specific characteristic? Can different genotypes affect the response to the environment?

• Sunbaking

• Bent trees are found near the sea

True or False

• The environment can change genotypes

• Dieting to lose weight is an example of environmental influence on a phenotype

• Identical twins can have small physical differences – this is due to different genotypes

• The phenotype represents the interaction of the genotype and its environment

• The genotype and the environment determine the maximum yield of a particular type of wheat

• The main reason that family members have similar characteristics is because they live together, and they are exposed to the same environment

• It is hard to study genetics because of the variation in phenotype caused by an uncontrollable environment

• The essential biological instructions passed from parents to offspring are known as the phenotype

• Different phenotypes are only ever due to different genotypes

Questions:

1. The allele for dimples (D) is dominant to the allele for no dimples (d).  A man heterozygous for dimples marries a woman who is also heterozygous for dimples. 

a.     What is the man’s genotype and the woman’s genotype?

b.     What is the man’s phenotype and the woman’s phenotype?

c.     Do a cross to determine all potential dimple genotypes and phenotypes for the offspring of this man and woman.

2. The allele for hitchhiker’s thumb (h) is recessive to straight thumb (H).  If a man and his wife are both homozygous recessive, will any of their offspring potentially have hitchhikers thumb?

a.       What is the man’s genotype and the woman’s genotype?

b.       What is the man’s phenotype and the woman’s phenotype?

c.       What genotype(s) must the offspring have in order to have the phenotypic trait of hitchhiker’s thumb?

d.       Do a cross to determine all potential hitchhiker’s thumb genotypes and phenotypes for the offspring of this man and woman.  Is it possible for any offspring of the F1 generation to have hitchhiker’s thumb?

3.)     In a certain breed of dogs, a gene (L) codes for hair length. The dominant trait is short hair and the recessive is long hair. Suppose a heterozygous female dog and a homozygous recessive male dog mate.

a.       What is the male dog’s genotype and the female dog’s genotype?

b.       What is the male dog’s phenotype and the female dog’s phenotype?

c.       What will be the genotypic ratio of the F1 generation?

d.       What will be the phenotypic ration of the F1 generation?

4. In fruit flies, the allele for normal wings (V) is dominant to the allele for short wings (v).  Suppose two fruit flies heterozygous for the trait are mated.

a.        What is the male fruit fly’s genotype and the female fruit fly’s genotype?

b.       What is the male fruit fly’s phenotype and the female fruit fly’s phenotype?

c.       What will be the genotypic ratio of the F1 generation?

d.       What will be the phenotypic ration of the F1 generation?

5. A genetic engineer is going to cross two watermelon plants to produce seeds for a spring planting.  He is breeding for size, and wants to have as many watermelons with the phenotype for long shape as possible.  In watermelons, the allele for short shape (R) is dominant to the allele for long shape (r).  Would crossing a watermelon homozygous recessive for the trait with a watermelon heterozygous for the trait give the most long watermelons possible?  Explain your answer using Punnett Squares.