Genetics and Inheritance

Key terms

• Gene: a section of DNA coding for a trait.
• Allele: an alternative form of a gene.
• Homozygous: two identical alleles (TT or tt).
• Heterozygous: two different alleles (Tt).
• Dominant: expressed even with one copy.
• Recessive: expressed only when two copies are present.

Mendel's work

Gregor Mendel crossed pea plants and found that traits are controlled by factors (now called genes) inherited in pairs. His law of segregation says alleles separate during gamete formation. His law of independent assortment says alleles of different genes sort independently.

Monohybrid crosses

Steps:

1. Assign letters (capital for dominant).
2. Write parent genotypes.
3. Write gametes.
4. Use a Punnett square.
5. State genotype and phenotype ratios.

Example: Tt x Tt.

Genotype ratio 1 TT : 2 Tt : 1 tt. Phenotype ratio 3 tall : 1 short.

Test cross

To find whether a tall plant is TT or Tt, cross it with tt. If all offspring are tall, the parent is TT. If half are short, the parent is Tt.

Incomplete dominance and codominance

• Incomplete dominance: heterozygote shows a blend, e.g. red x white four-o'clock flowers give pink.
• Codominance: both alleles expressed, e.g. blood group AB (IAIB).

ABO blood groups have three alleles (IA, IB, i). Group O is ii; group A may be IAIA or IAi.

Sex determination and sex linkage

Humans have 23 pairs of chromosomes. Females are XX and males XY. The father decides the sex of the child. Genes on the X chromosome show sex linkage. Haemophilia and colour blindness are X-linked recessive. A carrier mother (XHXh) and a normal father (XHY) can have a son with the condition (XhY) with 50% chance.

Genetic disorders in Kenya

• Sickle cell anaemia (HbSHbS): abnormal haemoglobin. Common in malaria regions of western Kenya and the coast. Carriers (HbAHbS) have a survival advantage against malaria.
• Albinism: recessive; lack of melanin, so affected people need sun protection.
• Haemophilia: poor blood clotting.

Genetic counselling helps couples understand risks before having children.

Mutations

A mutation is a sudden change in DNA. Causes include radiation, certain chemicals and errors in copying. Mutations may be harmful, neutral or beneficial and are the raw material of evolution.

Applications

Selective breeding in Kenya produced improved dairy cattle (Friesian x Zebu crosses), drought-tolerant maize and high-yielding tea clones. Biotechnology such as DNA fingerprinting is used in forensics and paternity tests.

Try it

• Cross a homozygous red-flowered plant with a white one and state the F1 and F2 ratios if inheritance is incomplete dominance.
• A couple are both carriers of albinism. What is the chance of an albino child?
• Draw a family pedigree for colour blindness over three generations.