Unit 2B Human Diversity & Change Inheritance Pedigrees

Symbols used in simple pedigrees Male Female Mating Unaffected individuals Affected individuals Offspring Twins I 1 2 Identical Non-identical II Deceased individuals 1 2 3 A pedigree is a diagram recording the ancestry of an individual

Autosomal pedigrees dominant or recessive? Aa Aa If two parents of similar appearance have an offspring who is different, the parents must have the dominant trait. The non-look-alike offspring is recessive. aa Aa Aa aa

Only a recessive trait can skip a generation aa Aa Aa Aa aa If neither of these events occurs in a pedigree then either trait could be dominant.

Aa aa ? Aa or AA? It is sometimes impossible to tell whether a dominant phenotype is homozygous or heterozygous.

Sex linked pedigrees dominant or recessive? XAXa XAy Notice that this is not the case with a female offspring. Males: Xay XAXa XAy If two parents of similar appearance have a male offspring who is different, the parents must have the dominant trait. The non-look-alike male offspring is recessive. Females: XAXA or XAXa Males: Xay or XAy

Is it sex linked or autosomal? XAXa Xay XAy XAXa Xay If only males inherit the trait it may be X-linked recessive. N.B. X-linked dominant pedigrees look very much like autosomal pedigrees.

Things that cannot happen in an X-linked pedigree A man with a dominant trait cannot have a daughter with the recessive trait. All his daughters will inherit his dominant gene. XAy XAX? XAX? XaXa A woman with a recessive trait cannot have a son with the dominant trait. All her sons will inherit her recessive gene. Xay Xay XAXA or XAXa XAy Two parents with the same trait cannot have a daughter with a different trait. XAXA or XAXa

Working out an X-linked pedigree XX Xy XX Xy XX Xy Xy XX 1. Apply the X and Y chromosomes

Working out an X-linked pedigree XX XAy XX XX XAy XAy Xay XX 2. Complete the male genotypes Males have only one gene so their phenotype is the same as their genotype

Working out an X-linked pedigree XAX XAy XAX XAX XAy XAy Xay XAX 3. Every female displays the dominant trait and must carry at least one dominant gene

Working out an X-linked pedigree XAXa XAy XAXa XAX? XAy XAy Xay XAX? 4. Deduce the genotypes of the females Remember, in some cases it will not be possible