LS 7B flash cards

The two alleles of a gene separate when gametes form, so each gamete carries just one of them.

It reflects homologous chromosomes parting in meiosis I, and explains the 3:1 ratio in a monohybrid F2.

1/4. Each parent passes on a with probability 1/2, and 1/2 × 1/2 = 1/4.

2/3. The dominant offspring come as AA, Aa and Aa, so two of every three are Aa.

Cross it with a homozygous recessive. Any offspring showing the recessive trait mean the parent was Aa.

If a large number of offspring all show the dominant trait, the parent is most likely AA.

Alleles of different genes go into gametes independently of one another, so one gene's allele does not predict the other's.

It holds for genes on different chromosomes, or far apart on the same one.

1/16. The chance of aa is 1/4 and of bb is 1/4, and 1/4 × 1/4 = 1/16.

The full dihybrid phenotype ratio is 9:3:3:1.

In anaphase I. Sister chromatids stay together until anaphase II.

The exchange of matching segments between non-sister chromatids of homologous chromosomes during prophase I.

It produces recombinant chromosomes carrying new combinations of alleles.

2²³, about 8.4 million. Each of the 23 homologous pairs can line up two ways at metaphase I.

A failure of homologs (in meiosis I) or sister chromatids (in meiosis II) to separate, giving gametes with an extra or a missing chromosome.

When a gamete carrying an extra chromosome 21 is fertilized, the result is trisomy 21 (Down syndrome).

Males have one X, so a single recessive allele is expressed. Females need two copies to show the trait.

1/2. A son gets his Y from his father and one of his mother's two X chromosomes, each equally likely.

Daughters all get the father's normal X, so none are affected, though half are carriers.

A heterozygote shows a phenotype in between the two homozygotes, such as pink flowers from red and white parents.

Crossing two heterozygotes then gives a 1:2:1 phenotype ratio, matching the genotypes.

A heterozygote fully expresses both alleles at once, as in AB blood type, where both A and B antigens appear.

When one gene's alleles mask or change the effect of a different gene.

In Labradors, an ee dog is yellow whatever it carries at the B gene, so a BbEe × BbEe cross gives 9 black : 3 brown : 4 yellow.

One gene affecting several different traits.

The sickle-cell allele changes red blood cell shape, which in turn causes anemia, pain and organ damage.

Type A (Iᴬi) or type B (Iᴮi), each with probability 1/2. Neither AB nor O is possible.

12 map units (cM). Recombination frequency is 120 / 1,000 = 12%, and 1% equals 1 map unit.

Crossovers happen at roughly random spots, so a longer stretch between two genes is more likely to hold one.

A crossover involves only two of the four chromatids, and extra crossovers can undo earlier ones, so at most half the gametes are recombinant.

50% is also what independent assortment gives, so genes far apart on one chromosome look unlinked in a cross.