Chapter 3 Heredity

Heredity: Passing of traits from parent to offspring.

Gregor Mendel “Father of Genetics”

Gregor Mendel  Lived from July 20, 1822 – January 6, 1884)  Austrian monk  Worked in monastery garden  Used pea plants to show that the inheritance of traits follows particular laws, which were later named after him

Trait – a characteristics of an organism Heredity – the study of how traits are passed from parents to offspring

Why Mendel used pea plants: 1.Quick reproduction and growth 2.Easily observed traits 3.Many observable traits 4.Easy cross pollination

Self pollinating Plants usually contain both male and female reproductive structures

During self pollination Pollen from anthers (male) is transferred to the stigma (female) Fertilization occurs when a sperm from the pollen travels through the stigma and enters the egg in the ovule.

True breeding plants – have offspring that always show the same form of the trait Cross fertilization – a process in which one plant fertilizes the egg in a flower of a different plant Pollen – tiny grains containing plant sperm cells

P generation – parental generation F1 generation – offspring in first cross = hybrids

Offspring from first cross are known as first generation. Dominant: The trait observed when at least one dominant allele for a characteristic is inherited. Recessive: A trait that is apparent only when two recessive alleles for the same characteristic are inherited,

Each parent donates one set of instruction to an offspring known as genes.

ALLELES Two forms of the same gene for every characteristic

Hybrids – the offspring of two different true breeding plants Monohybrid cross – a cross between two plants that differ in only one trait F2 generation – the offspring that result when 2 hybrid plants are crossed

Mendel’s P (Parental) Cross true breeder X true breeder short long Resulted in all long offspring (F1)

Mendel’s F1 (first filial) cross Cross pollinated 2 of the long offspring produced in the P generation F1 long X F1 long Results: 75% long and 25% short

Mendel’s conclusions: Gene – the factor that controls traits Allele – the possibilities of a gene (e.g. A or a) Simple dominance – one allele is dominant to a recessive allele

Mendel Vocabulary Dominant – the allele that masks any other allele when there are 2 alleles present (A in Aa) (symbolized by the first letter in the dominant trait’s name, always capital) Recessive – the allele that is masked by another allele (a in Aa) (symbolized by the first letter in the dominant trait’s name, always lower case)

Homozygous – having two identical alleles for a trait (AA or aa) (Mendel called this “true-breeding”) Homozygous dominant – having two dominant alleles for a trait (AA) (Mendel called this “true breeding dominant”) Homozygous recessive – having two recessive alleles for a trait (aa) (Mendel called this “true breeding recessive”) Heterozygous – having two different alleles for a trait (Aa) More Mendel Vocabulary

Phenotype – the visible traits of an organism (e.g. long or short) Genotype – the alleles that an organism carries (e.g. Aa or AA or aa)

Punnett square – a model used to represent crosses between organisms Example: What are the possible offspring of a cross between a homozygous dominant green plant (GG) and a homozygous recessive green plant (gg)?

Steps to doing a Punnett Square: 1. identify the gametes of the parents Example: What are the possible offspring of a cross between a homozygous dominant green plant (GG) and a homozygous recessive green plant (gg)? GG parent will produce all G gametes gg parent will produce all g gametes

Steps to doing a Punnett Square: 2. draw a square with 4 boxes Example: What are the possible offspring of a cross between a homozygous dominant green plant (GG) and a homozygous recessive green plant (gg)? GG parent will produce all G gametes gg parent will produce all g gametes

Steps to doing a Punnett Square: 3. put the gametes from one parent on the top of the box and the gametes from the other parent on the side of the box GG parent  all G gametes gg parent  all g gametes G G g g

Steps to doing a Punnett Square: 4. cross multiply to find the genotypes of the children GG g g G g

Steps to doing a Punnett Square: 4. cross multiply to find the genotypes of the children GgGg GgGg GgGg GgGg GG g g

Steps to doing a Punnett Square: 5. write the phenotypes of each child in the boxes GgGg GgGg GgGg GgGg GG g g Green

Steps to doing a Punnett Square: 6. calculate genotypic and phenotypic ratios of the offspring Gg Green Gg Green Gg Green Gg Green GG g g List all possible Genotypes: GG Gg gg List all possible Phenotypes: green yellow

Steps to doing a Punnett Square: 6. calculate genotypic and phenotypic ratios of the offspring Gg Green Gg Green Gg Green Gg Green GG g g Count how many of each: Genotypes: GG Gg gg Phenotypes: green yellow = 0/4 = 0% = 4/4 = 100% = 0/4 = 0% = 4/4 = 100% = 0/4 = 0%

Now try this one… What are the possible offspring of a cross between a pea plant which is heterozygous for green peas and a pea plant which is homozygous recessive for green peas?

Another example… What are the possible offspring of a cross between a mother and father who are both heterozygous for the ability to roll their tongues?

Incomplete Dominance Sometimes, there are two dominant alleles and no recessive alleles. Ex: flower color

In some flowers, red and white are both dominant. A red flower has the phenotype ______ and the genotype RR. A white flower has the phenotype ______ and the genotype WW. RED WHITE

What if you cross a RED (RR) flower with a WHITE (WW) flower? R R W W RWRW RWRW RWRWRWRW

What color are the offspring? RR W W RWRW RWRW RWRWRWRW RED WHITE + PINK

What is the phenotype of this flower? What is the genotype of this flower? PINK RW

So, when a trait is inherited by incomplete dominance, there are ____ possible phenotypes and ____ possible genotypes. 3 3 Red White Pink RR WW RW

Multiple Alleles In some cases, there are more than 2 possibilities. Ex: hair color, eye color, skin color

Blood Type 2 Dominant alleles – A and B 1 recessive allele – O GenotypePhenotype AAA AOA AB BBB BOB OOO

What if you cross a AO parent with a BO parent? AO B O AB BO AOOO