Genetics and Inheritance

Key terms

• Gene: a section of DNA coding for a trait.
• Allele: an alternative form of a gene.
• Dominant allele: expressed even when one copy is present (written as a capital letter, e.g. T).
• Recessive allele: expressed only when two copies are present (small letter, t).
• Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt).
• Genotype: the alleles an organism has. Phenotype: the observable trait.

Mendel's laws

Gregor Mendel studied garden peas. His work gives two laws:

1. Law of segregation: the two alleles of a gene separate during gamete formation so each gamete carries one allele.
2. Law of independent assortment: alleles of different genes are inherited independently of each other.

Monohybrid crosses

A monohybrid cross follows one trait. Steps:

1. Define the symbols.
2. Write parental genotypes.
3. Write the gametes.
4. Combine gametes in a Punnett square.
5. State genotype and phenotype ratios.

Worked example 1: Tall (T) is dominant over short (t). Cross Tt x Tt.

• Gametes: T, t and T, t
• Offspring: TT, Tt, Tt, tt
• Genotype ratio 1 TT : 2 Tt : 1 tt
• Phenotype ratio 3 tall : 1 short

Worked example 2 (test cross): A tall plant is crossed with a short one (tt). If all offspring are tall, the parent was TT. If half are short, the parent was Tt.

Incomplete dominance and codominance

• Incomplete dominance: the heterozygote shows an intermediate form, e.g. red x white flowers giving pink.
• Codominance: both alleles are fully expressed, e.g. blood group AB.

Blood groups

ABO blood groups are controlled by alleles IA, IB and i. IA and IB are codominant; both are dominant to i. Group O is ii. A woman of group A (IAi) and a man of group B (IBi) can have children of groups A, B, AB or O.

Sex determination and sex-linked traits

Humans have 23 pairs of chromosomes; the sex chromosomes are XX in females and XY in males. The father determines the sex of the child. Genes for red-green colour blindness and haemophilia are on the X chromosome, so they appear more often in males, who have only one X.

Genetic disorders

• Sickle cell anaemia: caused by a recessive allele producing abnormal haemoglobin; red cells become sickle-shaped and block capillaries.
• Albinism: recessive; lack of melanin.
• Haemophilia: sex-linked recessive; blood fails to clot.

Applications

Genetic counselling, selective breeding of dairy cattle and drought-tolerant maize varieties used in Kenya all apply these principles.

Try it

• In cattle, polled (P) is dominant over horned (p). Cross a heterozygous polled bull with a horned cow and give the ratios.
• Two normal parents have a child with albinism. Give the genotypes of the parents.
• Explain why a colour-blind father cannot pass the allele to his son.
• Draw a Punnett square for a cross between IAIB and ii.