Mendelian Genetics
How Traits Are Inherited
Mendel's Laws
Gregor Mendel (1860s) crossed pea plants to discover the laws of inheritance:
Law of Segregation: Each individual has two alleles for each gene (one from each parent). During gamete formation, the two alleles separate so each gamete carries only one. Upon fertilisation, offspring inherit one allele from each parent.
Law of Independent Assortment: Alleles for different genes (on different chromosomes) are inherited independently of each other. This applies to genes on different chromosomes (or far apart on the same chromosome).
Dominant and Recessive Alleles
An allele is a version of a gene. Convention: dominant allele = capital letter (A), recessive = lowercase (a).
- Homozygous dominant (AA): shows dominant trait
- Heterozygous (Aa): shows dominant trait (dominant masks recessive)
- Homozygous recessive (aa): shows recessive trait
Genotype = allele combination (AA, Aa, aa)<br>Phenotype = observable trait (what you see)
Punnett squares are used to predict offspring ratios. A monohybrid cross Aa × Aa gives 1 AA : 2 Aa : 1 aa (3:1 phenotype ratio).
Punnett Square: Aa × Aa
Cross two heterozygous parents:<br>``<br> A a<br>A: AA Aa<br>a: Aa aa<br>``<br>Genotype ratio: 1 AA : 2 Aa : 1 aa<br>Phenotype ratio: 3 dominant : 1 recessive
In a cross between two heterozygous individuals (Aa × Aa), what proportion of offspring will show the recessive phenotype?
The Punnett square gives AA, Aa, Aa, aa — only aa shows the recessive phenotype: 1 out of 4 = 25%.
Correct answer: 1/4 (25%)
Mendel's Law of Segregation states that:
Law of Segregation: the two alleles for a gene separate during meiosis so each gamete receives only one allele.
Correct answer: Allele pairs separate during gamete formation
A plant with genotype Tt crossed with a tt plant would produce what offspring ratio?
Tt × tt: gametes T and t from first parent, only t from second. Offspring: Tt and tt in equal proportion (1:1).
Correct answer: 1 Tt : 1 tt
A cross between two black guinea pigs (Bb × Bb) produces both black and white offspring. Explain this result using Mendelian genetics.
Heterozygous parents express the dominant trait but can pass the recessive allele to offspring.
Correct answer: Black coat (B) is dominant over white (b). The Bb × Bb cross gives: BB, Bb, Bb, bb. Only bb offspring (25%) show white coat because they lack any dominant B allele. The 75% with at least one B allele have black coat. This demonstrates the Law of Segregation.