Genes have loci on chromosomes, where chromosomes segregate through independent assortment.
Chromosomes: location of Mendel’s heritable factors
Fruits flies are convenient organisms used in studies: can produce 100’s offspring, eggs every 2 weeks, 4 pairs chromosomes, 3 pairs autosomes, 1 pair sex chromosomes
Wild type: phenotype most in population (red eyes: W+)
Mutant type: alternate to wild type (white eyes: w)
Correlating Allelic/Chromosome behavior
White eyed F x red eyed M
Expected to be all red but actually 50% red-eyed F and 50% white-eyed M
Sex-linked inheritance pattern (either sex chromosome)
Sex chromosomes
Autosomes: 22 pairs in humans
Homomorphic: same shape
Sex chromosomes: 1 pair
Heteromorphic: not morphologically similar, meiosis -> some regions synapse
Humans: 2 sex chromosomes X and Y
Males are heterogametic; XY genotype, half of sperm get X and other get Y
Females are homogametic; XX genotype, all eggs with X
Alternate systems of sex determinism like IW in reptiles
Haplodiploidy: not always chromosomal
X-linked genes
Sex-linked genes: either sex chromosome
Y-linked genes: on Y(few) most are related to sex determination(sex)
Only passed down male family members
X-linked genes: on X, are many genes as in autosome, not all determine sex
X-linked recessive disorders: more common in M than F
Males are hemizygous (carry only 1 X)
Alleles on X expressed, recessive alleles can’t be masked
Females are homozygous (2 X)
Recessive alleles can be masked by other X
X-inactivation
Females deal with 2 X chromosomes, are functionally hemizygous
Barr body: 1 X randomly inactivated during embryonic development
Inactive X condenses: inactivation involves DNA methylation(C-H)/histone proteins
Color in cats X-linked
If females are heterozygous (X^A X^a)
Some cells X^A is active, others have active X^a
Violations of independent assortment
Linked genes
Close genes tend to be inherited together
Linkage: genes inherited together
100% daughters have same alleles/gametes as parents, no recombination (if parents had DE and de gametes, so will daughters)
Genes are on same chromosome close together, violate law of independent assortment
If unlinked: 50% offspring like parents and 50% recombinants
Recombinant mechanisms
Parental type: gene combo parents have (AB/ab)
If no crossing over, 100% daughter same genes as parent
Genes recombined during prophase I
Parental type AB and ab
Recombination: form of gen. Combinations not seen in parents