(13) genes, chromosomes and human genetics

need to do pt2 of slides

3 factors that can diverge a trait from mendel’s ratio in complete dominance

1. the genes are close together on the same chromosome

2. the gene is located on a sex chromosome

3. the trait depends on chromosome number or structure

physical location of a gene

• locus: the position of a gene on a chromosome

• chromosomes carry genes in a linear order

independent assortment vs linked genes

independent assortment

• genes on different chromosomes

• assort independently during meiosis

• inheritance of one gene does not affect the other

• caused by random homolog orientation

linked genes

• genes on the same chromosome

• can be linked and inherited together

• crossing over can serparate linked genes

• genes farther apart of more likely to be separated by crossing over

• caused by physical association on one chromosome

recombination

• segments of DNA are exchanged between non-sister chromatids

• production of new alleles from crossing over

• occurs between homologous chromosomes during prophase of meiosis 1

• the farther apart the gene, the most likely crossing over occurs

parental type vs recombinant type

• parental type: offspring phenotype matching one of the original parental combinations

• recombinant type: offspring phenotype with a new combination of parental traits

drosophila (fruit fly) genetic notation

• wild type alleles are marked with a plus sign ie. pr+

  • mutant recessive alleles are marked without the plus sign ie. pr

parentals vs recombinants

parentals: preserve allele combination present in original parental chromosomes

recombinant: shows new combinations caused by crossing over in meiosis

• independently assorted genes: recombinant and parental classes are expected at similar frequencies

• linked genes: parental classes are more frequent

autosomes and autosomal linkage

autosomes: chromosomes that are not sex chromosomes

autosomal linkage

• inherited together more often than expected by independent assortment

• recombination can still generate new combinations of alleles

recombination frequency

(recombinant offspring / total offspring) x 100

• value estimates distance between two linked genes

• 1% = 1 map unit/mp

• maximum observable recombination frequency: = 50%

• < 50% suggests gene linkage

genetic maps

• shows the relative order and spacing of genes on chromosomes

• closer genes have lower recombination frequencies

• genes far apart on the same chromosome assort independently