Inheritance and meiosis: active recall

A gene is a DNA sequence associated with a functional product. Alleles are alternative versions of a gene, and the locus is its position on a chromosome. A diploid cell normally has two copies of an autosomal locus, but a population can contain more than two alleles of the gene.

No. Dominance describes how alleles contribute to a heterozygote’s phenotype. It does not determine their frequency, selective advantage or probability of being passed on from a heterozygote. With normal segregation, either allele can enter a gamete with equal probability.

After DNA replication, homologous chromosomes separate in the first division, and sister chromatids usually separate in the second. The products have one member of each homologous pair. Chromatid number and chromosome number are different quantities, so identify what is being counted.

During prophase I, exchange can occur between non-sister chromatids of homologous chromosomes. This produces new combinations of alleles along a chromatid. It rearranges existing alleles and is different from a mutation creating a new allele.

The orientation and segregation of each homologous chromosome pair can be independent of other pairs. Gametes can therefore receive different combinations of maternal and paternal chromosomes. Genes on the same chromosome are not automatically independent at the allele level.

With complete dominance, equal segregation, random fertilisation and no differential survival, the genotypes are expected in a 1 AA : 2 Aa : 1 aa ratio. Combining AA and Aa gives 3 dominant : 1 recessive. Other dominance relationships or biological effects can change the phenotype ratio.

Both alleles contribute a distinguishable effect in the heterozygote’s phenotype. The heterozygote is therefore not simply grouped with one homozygote as in complete dominance. A cross should label the actual phenotype associated with each genotype.

The standard AaBb × AaBb prediction assumes independent assortment, complete dominance at each locus and no interaction changing the phenotype categories, alongside equal gamete probabilities and survival. Linkage or epistasis can alter the prediction, so the parental genotype alone is not enough.

Their loci lie on the same chromosome, so allele combinations can be inherited together. Crossing over can produce recombinants, with frequency influenced by the distance and other features of the region. Use supplied linkage information rather than assuming four equal dihybrid gametes.

A conditional probability among sons uses only male outcomes as its denominator. A joint probability of an affected son among all offspring includes the probability of being male as well. Under equal sex probabilities, these can differ by a factor of two.

Epistasis involves an allele at one locus modifying or masking expression associated with another locus. Dominance concerns the relationship between alleles at the same locus. Interacting loci can therefore change a dihybrid phenotype ratio without changing the number of possible genotype combinations.

For a two-allele autosomal model with complete dominance, the recessive phenotype requires aa. Each allele copy has frequency q, giving q × q under the model’s random-mating assumption. q itself counts allele copies, not the proportion of recessive individuals.