Mendel’s Law of Heredity

Chapter 10, Section 1

The Father of Genetics

Gregor Mendel’s experiments founded many of the principles of Genetics we use today

He was a mathematician, he studied statistics

He was a teacher He was a gardener

Heredity = characteristics transmitted from 2 parents to their offspring

Trait = variant of a certain characteristic based on genetics

Mendel noticed plant height, flower position, pod color & appearance, seed color & appearance, and flower color

Gametes = sex cells Fertilization = process of uniting

female gamete & male gamete Zygote = fertilized cell Pollination = transfer of pollen

from male reproductive organ to the female reproductive organ (flower of plant)

Stamen – male part of plant. Anther contains pollen.

Pistil – female part of plant – contains the eggs.

Dominant & Recessive

Offspring get two copies of genes for a trait (ONE FROM MOM, ONE FROM DAD), sometimes one copy dominates over the other copy

Genes = found in chromosomes, made of DNA (they make you like you, they make peas look like peas!)

Allele = alternative forms of a gene Dominant = an allele that dominates

over the recessive allele Recessive = an allele that gets

masked if a dominant is paired with it

Mendel’s Monohybrid Crosses

Mendel crossed pea plants, testing one trait at a time

Ex: Flower Color– Mendel cross a

purebred purple flower pea with a purebred white flower pea

Hybrid = offspring of parents that have different forms of a trait

Purebred = produces offspring w/ the same trait when self-fertilized

P generation = parents F1 generation = offspring of the

parent generation, first generation F2 generation = offspring of a F1

mating with another F1, second generation

What happened to the white flowers in the F1 generation? Which color is the dominant color?

The recessive white color was “masked” in the F1 generation by the dominant purple color, but it reappeared again in the F2 generation

Each individual organism will have 2 alleles for a trait (remember, one from each parent)

Homozygous = an organism with 2 identical alleles for a trait

Heterozygous = an organism with 2 different alleles for a trait

Alleles for flower color are represented by letters:

P = dominant allele for purple flowers

p = recessive allele for white flowers

Be careful, representations are case sensitive!!!

Genotype = allele combination Phenotype = the way an

organism looks, trait that is expressed

What was the genotype of the purple flower from the P Generation?

PP What was the genotype of the white

flower from the P Generation? pp How do we figure the probability of the

genotypes of the F1 Generation? Monohybrid Cross (involves one trait)

Monohybrid Cross

Mendel’s Law of Heredity Cont.

Chapter 10, Section 1

Law of Segregation

Each individual has 2 alleles for each gene

During gamete (sex cell) production an individual separates its pairs of alleles

Each gamete gets one allele !!! Alleles pair up again when one

gamete joins with another gamete during fertilization

Law of Independent Assortment

Genes for different traits are inherited independently of one another

Ex: Pea plants can be tall (T) or short (t) and they can also have purple flowers (P) or white flowers (p)

Just because a pea plant has the dominant trait for height does not mean that it will have the dominant trait for flower color too

Dihybrid Crosses

Round seeds (R), wrinkled seeds (r) Purple flowers (P), white flowers (p) An individual whose genotype is RRPP

has what phenotype? Round seeds, purple flowers An individual whose genotype is rrpp

has what phenotype? Wrinkled seeds, white flowers An individual whose genotype is RrPp

has what phenotype? Round seeds, purple flowers

2 characteristics per cross Punnett Square has 16 boxes

Method to organize alleles on a Punnett Square

F first O outter I inner L last