Monohybrid crosses, codominance and sex linkage

Last time

Before we start: last time, in Meiosis and how genes make phenotypes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Predicting the offspring

Two brown eyed parents can have a blue eyed child. That is not luck, it is arithmetic, and you can predict it with a small square. The same square handles blood groups, where more than two alleles are in play, and conditions carried on the X chromosome. The first step is the vocabulary.

Key terms

A gene is a length of D N A that codes for a polypeptide. An allele is one version of a gene. Alleles occupy the same position, or locus, on homologous chromosomes. The genotype is the alleles an organism has. The phenotype is the observable feature. Homozygous means two identical alleles, heterozygous means two different alleles. A dominant allele is expressed in the phenotype even when only one copy is present. A recessive allele is expressed only when two copies are present.

A monohybrid cross

Cystic fibrosis is recessive. Two carriers, both genotype Cc, have children. Each parent makes two kinds of gamete, C and c, in equal numbers. The gametes combine at random. The offspring are one C C, two C c and one c c: a genotype ratio of one to two to one. The phenotype ratio is three unaffected to one with cystic fibrosis. Each child has a one in four chance of being affected, which is 25 percent.

A past-paper example

February March 2024, Paper 42, question 3 part a. Explain the meaning of the terms homozygous and recessive. Homozygous means having two identical alleles of a gene. Recessive means the allele is only expressed in the phenotype when there is no dominant allele present. So a person with cystic fibrosis is homozygous recessive, c c.