Codominance, sex linkage and pedigree diagrams

Last time

Before we start: last time, in Pregnancy, the placenta, STIs and HIV, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Not always dominant

Some features do not follow the simple dominant and recessive rule. In blood group A B, both alleles show at once. Today: codominance and blood groups, sex determination and sex linkage, and how to read a family pedigree.

Codominance

Codominance is when both alleles in a heterozygous organism affect the phenotype. Human A B O blood groups have three alleles: I A, I B and I o. I A and I B are codominant with each other, and both are dominant to I o.

Blood group genotypes

Here are the genotypes for each group. Groups A and B can each have two genotypes. Group A B is I A I B, both showing. Group O must be I o I o.

A blood group cross

A woman with group A B and a man with group B, heterozygous, have a child. Her gametes are I A or I B. His are I B or I o. The children could be A B, A, B or B: a fifty percent chance of group B.

Sex determination

Females have two X chromosomes: X X. Males have one X and one Y: X Y. Every egg carries an X. Half the sperm carry X and half carry Y. So the father's sperm decides the sex, and each child has a fifty percent chance of being a boy.

Sex linkage

A sex-linked characteristic is one where the gene is on a sex chromosome, usually the X. Red-green colour blindness is caused by a recessive allele on the X chromosome. Males have only one X, so one recessive allele is enough. That is why it is more common in males. A female needs two copies. With one, she is a carrier with normal vision.

Pedigree diagrams

A pedigree shows how a feature is passed through a family. Squares are males, circles are females. Shaded symbols show people with the feature. If two unaffected parents have an affected child, the feature must be recessive, and both parents are carriers.