AP Bio Genetics Part 2

Cellular Mosaic Pattern in Females

After an X chromosome is inactivated in a particular cell, all miotic descendants of that cell will have same inactive X -Female heterozygous for sex-linked trait- half of cells express one allele, other will express the alternate allele

Why are recombinants rare?

Crossovers between two genes that are close together are not very common -frequency of crossovers between two genes depends on the distance between them -short distance is small target for crossover events, with few such events taking place

T.H. Morgan

Crucial chromosome theory verifying experiments began when he found a mutation in gene affecting eye color. -Mutation made eyes white not red -Found that eye color gene was inherited in different patterns by male and female -gene inherited in the same pattern as X chromosome

Chromosomal Theory of Inheritance

States that genes are found at specific locations on chromosomes and that the behavior of chromosomes during meiosis can explain why genes are inherited according to Mendel's Laws -Genes lie on chromosomes -Genes are stretches of DNA that specify proteins

Law of Segregation

The two alleles for each gene separate during gamete formation 1) R and r alleles segregate at anaphase 1, yielding two types of daughter cells for this locus 2) Each gamete gets one long chromosome with either R or r allele

Homologous Recombination

When genes are on the same chromosome but very far apart, they assort independently due to crossing over -when genes are far apart, crossing over happens enough that all types of gametes are produced

Role of crossing over in linked genes

accounts for recombination of linked genes and set of proteins causes exchange of corresponding segments of maternal and paternal chromatids

Morgan's Drosphila Experiment

resulting flies had a much higher proportion of the combinations of traits seen in P generation flies that would be expected if the two genes assorted independently -concluded body color and wing size are usually inherited together in specific combinations because the genes for these characteristics are near each other on the same chromosome

Difference between X and Y chromosomes

-Y is smaller than X -short segments of Y are the only regions homologous with corresponding regions of X -sperm carries X or Y for fertilization -SRY- sex determining region of Y, gene -Very few disorders transferred from father to son on Y, rare absence of some Y genes

X-linked disorders

-color blindness -Duchene muscular dystrophy- weakened muscles -Hemophilia- absence of proteins for clotting blood

Reasons for high expression of disorders in males

-males only have one locus, hemizygous -males receiving recessive trait allele from mom will express that trait

Observations that support the Theory

1) Chromosomes (like Mendel's genes) come in matched homologous pairs in an organism. For both genes and chromosomes, one member of the pair comes from the mother and one from the father. 2) Members of homologous pairs separate during meiosis, so each sperm or egg receives just one member. Process mirrors segregation of alleles into gametes in Mendel's Law of Segregation 3) Members of different chromosome pairs are sorted into gametes independently of one another in meiosis, just like the alleles of different gene's in Mendel's Law of Independent Assortment

Law of Independent Assortment

Alleles of genes on non-homologous chromosomes assort independently during gamete formation 1) Alleles at both loci segregate in anaphase 1, yielding 4 types of daughter cells depending on chromosome arrangement 2) Each gamete gets a long and short chromosome in one or four allele combinations

Sex Inheritance Pattern

Genes must lie on or be closely related to the X chromosome -unexpected eye color produced through nondisjunction of sex chromosomes in meiosis -Morgan found mutations on other genes that weren't inherited in a sex-specific pattern -genes are born on both sex and non-sex chromosomes

Assort Independently

When gametes are on separate chromosomes, or very far apart on the same one -When the genes go into gametes, the allele received for one gene doesn't affect the allele received for the other -genes on separate chromosomes assort independently because of the random orientation of homologous chromosome pairs during meiosis

Linked

When genes are close together on the same chromosome -means that alleles, or gene versions, already together on one chromosome will be inherited as a unit more frequently than not -crossing over still occurs but outcome of gamete types produced is different -instead of assorting independently, genes stick together during meiosis -alleles of the genes that are already together on a chromosome tend to be passed as a unit to gametes

Common Types of Gametes

contain parental configurations for alleles that were already together on the chromosome in the organism before meiosis -on chromosome it got from parents

Rare Types of Gametes

contain recombinant configurations of alleles that can only form if a recombination event (crossing over) occurs between the genes

Recombination Frequency

estimates the relative distance apart on chromosome -closer together - have very few recombination events and tightly linked -further apart - have more recombination events and less tightly linked

Sex-linked genes

gene located on either sex chromosome -males and females inherit different number of X chromosomes leading to a pattern of inheritance different from that produced by genes located on autosomes

Genetic Linkage

genes that are sufficiently close together on a chromosome will tend to stick together and the versions (alleles) of those genes that are together on a chromosome will tend to be inherited as a pair -linked genes will have genetic crosses involving those genes will lead to ratios of gametes and offspring types that aren't what we'd predict from Mendel's law of independent assortment

Unlinked

when genes are found on different chromosomes or are far apart on the same chromosome -assort independently