Mendelian Genetics, Variation on Mendelian Theme, Nature vs Nurture

Who was Gregor Mendel?

A monk and scientist who discovered the basic laws of inheritance using pea plants.

What plant did Mendel use in his experiments?

Garden pea (Pisum sativum).

What is a monohybrid cross?

A genetic cross between two organisms involving one trait with two alleles.

What is Mendel's Law of Segregation?

Each individual has two alleles for each trait, which segregate during gamete formation.

What is Mendel's Law of Independent Assortment?

Alleles of different genes assort independently during gamete formation.

What is a dominant trait?

A trait that is expressed in the F1 generation, masking the recessive allele.

What is a recessive trait?

A trait that is hidden in the F1 generation but reappears in the F2.

What is a genotype?

The genetic makeup of an organism (e.g., Ss, ss).

What is a phenotype?

The observable characteristics of an organism.

What does homozygous mean?

Having two identical alleles (e.g., SS or ss).

What does heterozygous mean?

Having two different alleles (e.g., Ss).

What were the results of Mendel's F2 generation in a monohybrid cross?

A 3:1 ratio of dominant to recessive traits.

What is a dihybrid cross?

A cross involving two traits (e.g., seed shape and color).

What is the F2 phenotypic ratio in a dihybrid cross?

9:3:3:1

How do Mendel's laws relate to meiosis?

Meiosis explains the segregation and independent assortment of alleles.

What is the difference between genotype and phenotype?

Genotype is the gene combination; phenotype is the expression.

What is particulate inheritance?

The idea that genes are inherited as discrete units, not blended.

Why was Mendel's work ignored at first?

It preceded the discovery of DNA and wasn't understood until rediscovered in the 1900s.

What are alleles?

Alternative forms of a gene found at the same locus.

What is the relationship between diploid and haploid?

Diploid = 2 sets of chromosomes; haploid = 1 set (in gametes).

What is incomplete dominance?

A heterozygote shows an intermediate phenotype (e.g., pink carnations).

What is codominance?

Both alleles are expressed (e.g., AB blood group).

What is a lethal allele?

An allele that causes death when homozygous (e.g., MM in Manx cats).

What are multiple alleles?

More than two possible alleles for a gene (e.g., ABO blood types).

What is epistasis?

One gene masks or modifies the expression of another gene.

What are examples of epistasis?

Sweet pea color (A and B needed for pigment), Labrador coat color (B/b and E/e).

What is penetrance?

The percentage of individuals with a genotype that show the expected phenotype.

What is expressivity?

The degree or intensity with which a phenotype is expressed.

What causes incomplete penetrance?

Environmental effects, genetic interactions, age, or epigenetics.

What is extranuclear inheritance?

Inheritance through organelle DNA (mitochondria, chloroplasts).

What is maternal inheritance?

Traits inherited through the mother, often due to mitochondrial DNA.

What is linkage?

Genes on the same chromosome tend to be inherited together.

What is a chiasma?

A crossover point where genetic material is exchanged during meiosis.

What is a recombination frequency?

The percentage of offspring showing recombination — used to map genes.

What is a map unit or centiMorgan (cM)?

1% recombination = 1 map unit.

Why did Mendel not detect linkage?

His traits had ~50% recombination rates (no close linkage).

What is Lyonisation?

X chromosome inactivation in females, forming a Barr body.

What is a manifesting carrier?

A female carrier of an X-linked trait who shows symptoms due to skewed X-inactivation.

What are examples of X-linked disorders?

Haemophilia, Duchenne muscular dystrophy, red/green color blindness.

What did Thomas Hunt Morgan discover?

That genes are located on chromosomes (e.g., white eye gene in flies on X).

What is polygenic inheritance?

When multiple genes contribute to a single trait (e.g., human height).

What is the role of environment in polygenic traits?

It modifies expression, leading to a continuous range of phenotypes.

What is the "norm of reaction"?

The range of phenotypes a single genotype can produce under different environmental conditions.

What is heritability (h²)?

A measure of how much of a trait's variation is due to genetic differences.

What does h² > 0.5 mean?

The trait is more influenced by genes than environment.

What does h² < 0.5 mean?

The trait is more influenced by environment than genes.

How is heritability tested?

Using twin studies, especially identical twins raised apart.

Why are twin studies limited?

Small sample sizes, shared environments, and ethical concerns.

What is eugenics?

The historical idea of improving genetics by controlling reproduction, often with unethical implications.

Who promoted early eugenics?

Sir Francis Galton.

Why is separating nature and nurture difficult?

Because both genes and environment always interact to shape phenotype.

What does genetics tell us about identity?

Genes explain where we come from, but not who we are — identity is shaped by both DNA and experience.