Mendel and patterns of inheritance
Why do children resemble their parents but not exactly? Gregor Mendel's pea experiments in the 1860s uncovered the basic rules.
Mendel's experiments
Mendel bred pea plants with clear-cut traits, such as purple or white flowers, and counted the offspring over generations. Crossing pure purple with pure white gave all purple offspring, but white reappeared in about a quarter of the next generation. He concluded that traits are passed on as discrete units, now called genes.
Genes and alleles
A gene is a stretch of DNA that affects a trait; alleles are different versions of a gene. You inherit one allele of each gene from each parent. If both alleles are the same you are homozygous; if they differ you are heterozygous.
Dominant and recessive
A dominant allele shows its effect even with one copy; a recessive allele shows only when two copies are present. Writing purple as P and white as p, plants that are PP or Pp are purple, and only pp plants are white. Recessive alleles can hide for generations in carriers.
Genotype and phenotype
The genotype is the set of alleles an organism has (PP, Pp, or pp); the phenotype is the observable trait (purple or white). Different genotypes can give the same phenotype. The environment can also affect phenotype, as with height and nutrition.
Punnett squares
A Punnett square predicts offspring ratios. For Pp x Pp, list each parent's possible gametes (P or p) along the sides and fill in the boxes: PP, Pp, Pp, pp. That gives a 1 : 2 : 1 genotype ratio and a 3 : 1 purple-to-white phenotype ratio, matching Mendel's counts.
Mendel's laws
The law of segregation says the two alleles of a gene separate when gametes form, so each gamete carries one. The law of independent assortment says alleles of different genes sort independently, giving a 9 : 3 : 3 : 1 ratio in a cross involving two traits, as long as the genes are on different chromosomes.
The test cross
To find whether a purple plant is PP or Pp, cross it with a white (pp) plant. If any offspring are white, the purple parent must be Pp. If all of many offspring are purple, it is almost certainly PP. Breeders use this to check breeding stock.
Worked example
Two parents are carriers (Aa) of a recessive condition. Each child has a 1 in 4 chance of being affected (aa), 1 in 2 of being a carrier (Aa), and 1 in 4 of being unaffected and not a carrier (AA). Each child's chances are independent: having one affected child does not change the odds for the next.
Key takeaways
Genes come in alleles, one from each parent. Dominant alleles mask recessive ones. Genotype is the allele combination; phenotype is the trait. Punnett squares predict ratios such as 3 : 1. Alleles segregate into gametes, and different genes assort independently when on separate chromosomes.
Flashcard
DNA segment affecting a trait
Check your understanding
Two heterozygous purple plants (Pp) are crossed. What fraction of offspring is expected to be white?