Introduction to Mendelian genetics

What is an allele?

Is a variant of a gene controlling the same trait

Alleles usually occur in pairs

Can be dominant or recessive depending on how they are expressed

What is meant by homozygous?

An individual carrying two copies of the same allele for a given gene, i.e., BB or bb

Dominant = B

Recessive = b

What is meant by heterozygous?

An individual carrying one copy of each of two different alleles for a given gene i.e., B and b

What is a genotype?

Genetic makeup

What is a phenotype?

Physical expression (how the individual looks)

What is a dominant allele?

The allele/trait that is expressed in a heterozygous individual, ‘overriding’ the recessive allele/trait

What is a recessive allele?

The allele/trait that is not expressed in a heterozygous individual, phenotype can appear to disappear in one generation and reappear in a subsequent generation

What did Mendel want to try and figure out?

He wanted to know how traits were transferred from one generation to the next - and why some skip a generation

What was involved in Mendel’s experiments?

He studied 7 traits in peas

He noted that some traits appeared to be dominant over others, which were recessive

Only these dominant traits would be visible in the next generation

However, if these plants were crossed with each other, some of the offspring would regain the recessive trait

He also noted that the traits segregated with very specific ratios

What is involved in Mendel’s first law? Or the law of segregation

One pair of chromosomes

A heterozygous individual with two different alleles for the same gene

Which chromosome (and therefore allele) ends up in which cell at the end of meiosis 1 is completely random

Which chromosome (and therefore allele) ends up in which gamete at the end of meiosis 2 is completely random

For any single gene, the alleles segregate independently into gametes, such that any gamete has an equally likely chance of inheriting either allele

The separation of allele pairs during the formation of gametes (sperm and egg cells) so that each gamete receives only one allele for each trait

What is involved in Mendel’s second law? Or the law of independent assortment/random assortment

Two pair of chromosomes

A heterozygous individual with two different alleles for the same gene (A) and two different alleles for the same gene (B)

For any pair of genes, the alleles of one gene will sort into gametes independently of the alleles of the second gene

Which allele of gene A a gamete receives does not affect which allele of gene B that same gamete receives

All gametes are equally likely

What is a punnet square and what is it used for?

A square diagram used to predict the genotypes of a particular cross

Used to determine the probability of an offspring having a particular genotype

Summary of all possible combinations of maternal alleles with paternal alleles

What is the definition of purebreeding?

Homozygous with homozygous

Purebred crossed with a purebred = predictable results

Every gamete is identical

What is a heterozygote (F1)?

F1= product of crossing two DIFFERENT pure-breeding parents

Monohybrid cross (single gene cross)

e.g., parent 1 AA crossed with parent 2 aa (single gene)

What is a hybrid cross?

Heterozygous with heterozygous

F2 x F1

i.e., one gene Aa x Aa or two gene AaBb x AaBb

One gene hybrid cross = Aa x Aa (MONOhybrid cross)

Two gene hybrid cross = AaBb x AbAb (DIhybrid cross)

What is a test cross?

Heterozygous with homozygous RECESSIVE

i.e., one gene Aa x aa or two gene AaBb x aabb

What is a monohybrid cross and what is the phenotypic ratio of this type of cross?

One gene cross; parents = parent 1 Bb x parent 2 Bb

Progeny = 3 dominant and 1 recessive

Phenotypic ratio = 3:1

What is a dihybrid cross and what is the phenotypic ratio of this type of cross?

Two gene cross; parents = AaBb x AaBb

Phenotypic ratio = 9:3:3:1

9 - dominant A & B

3 - dominant A, recessive b

3 = recessive a, dominant B

1 - recessive a & b

What is a one gene test cross and what is the phenotypic ratio of this type of cross?

One gene cross; parents = Aa x aa

Parent 1 gametes = Aa

Parent 2 gametes = aa

= two dominant and two recessive progeny

Phenotypic ratio= 1:1

What is a two gene test cross and what is the phenotypic ratio of this type of cross?

Two gene cross; parents = AaBb (heterozygous) x aabb (homozygous recessive)

Parent 1 gametes = AB, Ab, aB, ab

Parent 2 gametes = ab

All possible options can be made in progeny

Phenotypic ratio = 1:1:1:1

What is a reciprocal cross?

This type of cross involves reversing the sexes of the parents in a breeding experiment to assess the role of parental sex on inheritance patterns. It essentially involves two crosses: one where a trait is passed from male to female, and another where the same trait is passed from female to male. This technique helps determine if traits are sex-linked, maternally inherited, or influenced by cytoplasmic factors.