3.4. Inheritance

Gregor Mendel

Considered the father of genetics due to his research with pea plants to study inheritance

Monohybrid cross

The crossing of two individuals that only differ in one trait

Mendel’s experiment

Performed monohybrid crosses between pea plants

* Transferred the pollen from one variety to the reproductive parts of another variety
* Experimented seven different pairs of characteristics, leading to very reliable results
* Each crossing was repeated with many pea plants

What were the traits tested by Mendel?

Flower color, plant height, seed color, seed shape, pod color, pod shape and flower position

What did Mendel observe in each crossing?

* All plants in the F1 generation had the same characteristic as one of the parent plant
* Pea plants in F2 generation had characteristics of both parent plants in a 3:1 ratio

Mendel’s discoveries

* Traits do not disappear between generations, but are rather not expressed
* Each trait is coded by alleles, which can be dominant or recessive

Segregation

The separation of two alleles of a diploid nucleus into two haploid nuclei

Law of segregation

The allele expressed by the offspring is determined by whether they inherited the dominant or recessive form of allele

Genotype

The combination of alleles that determine any given trait

Phenotype

The observable characteristics of an organism

Dominant allele

Only one needs to be inherited for the characteristics to be expressed

Recessive allele

Two need to be inherited for the characteristic to be expressed

What happens when there is only one recessive allele?

It will remain hidden and the dominant characteristic will be expressed

Homozygous

An individual with two identical alleles

Types of homozygous

Homozygous dominant: two copies of dominant allele

Homozygous recessive: two copies of recessive allele

Heterozygous

An individual with two different alleles

Punnett grid

* Illustrates the possible genotypes and phenotypes of offspring resulting from a genetic cross
* Predicts the probability of offspring displaying a certain genotype or phenotype

Co-dominance

When both alleles for a trait are equally expressed in a heterozygote

* Both alleles are considered dominant

Examples of co-dominance

Mirabilis jalapa

* Allele Cw → white flower
* Allele CR →  red flower
* Offspring will be CRCW; some parts are red and some are white

Palomino horse

* Allele HB → chestnut horse
* Allele HW → white horse
* Offspring will be HBHW; some hairs will be chestnut and some white

ABO blood group (genotypes and phenotypes)

Gene for blood type is I, and has three common alleles: I^A, I^B and i

Why are I^A and I^B dominant?

All three alleles cause the production of a glycoprotein in the membrane of red blood cells

* I^A alters this glycoprotein by addition of acetyl galactosamine. Since this altered glycoprotein is absent in people lacking I^A (type B and O), if exposed to it, they will make anti-A antibodies
* I^B alters this glycoprotein by addition of galactose. Since this altered glycoprotein is absent in people lacking I^B (type A and O), if exposed to it, they will make anti-B antibodies
* Allele i is recessive because it does not alter the glycoprotein. Thus, heterozygous and homozygous dominant give the same phenotype

Why are I^A and I^B co-dominant?

The genotype IAIB causes the glycoprotein to be altered by addition of acetyl-galactosamine and galactose. As a consequence, neither anti-A nor anti-B bodies are produced. This genotype therefore gives a different phenotype to IAIA and IBIB so the alleles IA and IB are co-dominant

Incomplete dominance (+ example)

When neither allele is fully expressed and rather an intermediate expression of a trait is seen (e.g. snapdragon)