Inheritance

Biology · D Continuity and change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

The genotype is the combination of alleles an organism has. The phenotype is its observable traits, which come from the genotype and the environment. A dominant allele shows in the phenotype even when only one copy is present. A recessive allele shows only in a homozygote. A Punnett grid lists the gametes of each parent and all the possible combinations in the offspring.

Some patterns are different. In codominance both alleles show in the heterozygote. In incomplete dominance the heterozygote is intermediate. Genes on the X chromosome are sex-linked, so males, who have one X, show a recessive allele more often. Traits controlled by many genes vary continuously.

Rules to remember

Aa × Aa gives 1 AA : 2 Aa : 1 aa, which is 3 : 1 in phenotypes. Aa × aa gives 1 : 1.
ABO blood groups: I^A and I^B are codominant, and both are dominant to i. Group O is ii and group AB is I^AI^B.
Sex: XX is female, XY is male. A father gives his X to every daughter and his Y to every son.
Haemophilia: recessive allele on the X chromosome (X^h). X^HX^h is a carrier female and X^hY is an affected male.
Pedigree: two unaffected parents with an affected child means the allele is recessive and both parents are carriers.
Phenylketonuria (PKU): an autosomal recessive condition. Phenylalanine cannot be converted to tyrosine.
Box-and-whisker plot: interquartile range (IQR) = Q3 − Q1. An outlier lies more than 1.5 × IQR above Q3 or below Q1.

Common mistake

Students write sex-linked alleles without the chromosome, for example Hh for a male. Always write them on the X (X^H, X^h) and write Y with no allele, then the difference between sons and daughters is clear.

Worked example

A woman who is a carrier of haemophilia (X^HX^h) has children with a man who has haemophilia (X^hY). What is the probability that a daughter has haemophilia?

Mother's gametes: X^H or X^h. Father's gametes: X^h or Y.
Offspring: X^HX^h, X^hX^h, X^HY, X^hY, each with probability ¼.
Daughters are X^HX^h (carrier) or X^hX^h (haemophilia), in equal numbers.

Answer: 0.5 (1 in 2 daughters). For any child, the probability of being a daughter with haemophilia is 0.25.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

D3.2 Inheritance

Keep going

← Reproduction Homeostasis →Take a Biology test