Mendel's Laws of Inheritance

Genetics

branch of biology that deals with heredity and variation of organisms

Chromosomes

they carry the hereditary information (genes)

Gregor Johann Mendel

• He developed the principles of genetics in mid 19th century

• Austrian monk

• He experimented with pea plants by crossing various strains and observing the characteristics of their offspring for eight years

Pea plants

What did Mendel experiment with?

• easy to cultivate

• short generation time

• cross-pollination by hand

Why did Mendel experiment with garden peas?

7

How many traits/characteristics of pea plants did Mendel look at?

Artist’s brush

What did Mendel use to cross two different pea plants?

True breeding

When the plants self-pollinate, all their offspring are of the same variety

Hybridization

Mating, or crossing, of two varieties

Monohybrid cross

A cross between two parents that breed true for different versions of a single trait

P generation

Which generation are the true breeding parents?

F1 generation (first filial)

Which generation is the hybrid offspring of the P generation?

F2 generation (second filial)

Which generation is the offspring from the self-fertilization of the F1 hybrids?

Self pollination

involves having the pollen (male sperm) be directly deposited on the female section of the flower

Cross pollination

requires the removal of the male stamen on 1st flower and transferring the pollen from a different flower to the first one

Theory of Inheritance

Mendel’s experiments helped him formulate what theory?

Inheritance

involves reshuffling of genes from generation to generation

One-Trait Inheritance

• Mendel performed cross-breeding experiments between true-breeding plants

• Chose varieties that differed in only one trait (monohybrid cross)

• Performed reciprocal crosses

• Formulated Law of Segregation

Reciprocal crosses

individuals pollinate one another

Law of Segregation

• Each individual has two factors for each trait

• The factors segregate during gamete formation

• Each gamete contains only one factor from each pair of factors

• Fertilization gives each new individual two factors for each trait

• When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene

Alleles

alternative forms of a gene that code for a trait

2

How many alleles are in each trait?

Homologous chromosomes

paired chromosomes that have alleles for same genes at specific loci

Law of Dominance

• Dominant allele (capital letter) masks the expression of the recessive allele (lower-case)

• Alleles occur on a homologous pair of chromosomes at a particular gene locus (location of gene on the chromosome)

  • Homozygous = identical alleles (TT, tt) • Heterozygous = different alleles (Tt)

Genotype

Refers to the alleles an individual receives at fertilization • If alleles are identical, it is homozygous
• If alleles are different, it is heterozygous

Phenotype

Refers to the physical appearance of the individual

Punnet Square

a square grid used to calculate the frequencies of the different genotypes and phenotypes among the offspring of a cross

Dihybrid cross

Matings that involve parents that differ in two genes (two independent traits)

Law of Independent Assortment

“Pairs of alleles for different traits separate independently of one another during gamete formation (meiosis).”

• In other words, the inheritance of one trait has no influence on the inheritance of another trait

• Genes get shuffled – these many combinations are one of the advantages of sexual reproduction