Practice alleles, segregation, genetic crosses, inheritance patterns, and probability.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
A gene influences a product or trait; alleles are alternative versions at that gene’s locus. Genotype is the allele combination. Phenotype is an observable feature shaped by genotype and environment.
In a simple complete-dominance model, AA and Aa can share a phenotype even though their genotypes differ. Lowercase does not mean an allele is weak or rare.
For Aa × Aa under complete dominance, what fraction of offspring are aa, and what is the genotype ratio?
1/4 are aa. The genotype ratio is 1 AA : 2 Aa : 1 aa.
Each parent contributes a with probability 1/2. Multiply independent contributions: 1/2 × 1/2 = 1/4. The dominant-to-recessive phenotype ratio is 3:1.
Mendel’s law of segregation says the two alleles for a gene separate when gametes form. A heterozygous Aa individual produces A and a gametes in equal proportions under the simple model.
The physical basis is the separation of homologous chromosomes in meiosis. A normal gamete carries one allele per gene because it has one chromosome from each homologous pair.
A dominant-phenotype plant could be AA or Aa. What cross can help distinguish them?
Cross it with a homozygous recessive plant, aa.
AA × aa produces only Aa offspring. Aa × aa predicts equal proportions of Aa and aa. A recessive offspring reveals that the tested parent carried a recessive allele.
For two Aa parents, their first three children are unaffected by an aa recessive disorder. What is the probability their next child is aa?
1/4, or 25%, under the simple Mendelian model.
Each fertilization is a new event. Earlier outcomes do not make a recessive child “due.” The parental allele probabilities are unchanged.
In incomplete dominance, the heterozygote has an intermediate phenotype, such as pink flowers from red and white alleles. In codominance, both allele products are distinctly expressed, as in AB blood type.
These patterns change how heterozygotes look; they do not cancel allele segregation. Phenotype ratios can therefore differ from the familiar 3:1 pattern.
For AaBb × AaBb, what is the probability of aabb if the genes assort independently?
1/16.
The probability of aa is 1/4, and the probability of bb is 1/4. Multiply: 1/4 × 1/4 = 1/16. Linked genes may not follow this independent-assortment model.
Pleiotropy means one gene affects multiple traits. Polygenic inheritance means multiple genes contribute to one trait. Environment can influence many phenotypes too.
Human height is influenced by many genes and by factors such as nutrition. A trait that varies continuously is often poorly described by one dominant-versus-recessive pair.
Why is an X-linked recessive allele often expressed in a person with one X and one Y chromosome?
They usually have only one copy of most X-linked genes, so there may be no corresponding dominant allele on the Y to mask it.
This is hemizygosity, not greater strength of the recessive allele. In the usual XX/XY pattern, an XY parent passes the X to daughters and the Y to sons.