Mendelian Genetics

Bio 181 – Week 1

Goals • Describe simple genetic

dominance, incomplete dominance, and lethal inheritance.

• Describe possible genotypes for some of your personal traits inherited as dominant and recessive genes.

• Explain the importance of Mendel’s Law of Segregation and Law of Independent Assortment

• Distinguish between an organism’s phenotype and genotype.

Today

• 17.1 – Determine genotypic and phenotypic ratios for albinism

• 17.2 – Determine color ratios for corn plants • Incomplete dominance • 17.3 – Determine blood type for ABO system • 17.4 – Determine Rh • Sex-linked inheritance • 17.5 – Analyze a pedigree of inheritance of cystic fibrosis • 17.6 – Analyze a pedigree of inheritance of Huntington’s

disease

• Work in groups of 2-4, complete handout and turn in before leaving.

Gregor Mendel

• “Experiments in Plant Hybridization” – published in 1866, noticed in 1900.

• Replaced “blending theory” of inheritance with “particulate theory”

Gregor Mendel – Experimental Design

• Used the garden pea, Pisum sativum as a model organism

– Easily recognizable traits: e.g., flower color, seed shape

– Could control which individuals were mating (removed male reproductive organ before pollen formed)

• Obtained “pure lines” or “true-breeding lines” from breeders in which a trait was always seen

1

5

4

3

2

Removed stamens

from purple flower

Transferred sperm-

bearing pollen from

stamens of white

flower to egg-

bearing carpel of

purple flower

Parental

generation

(P)

Pollinated carpel

matured into pod

Carpel

(female)

Stamens

(male)

Planted seeds

from pod

Examined

offspring:

all purple

flowers

First

generation

offspring

(F1)

Mendel’s Experiments with a Single Trait

• Blending inheritance hypothesis: claimed that traits observed in a mother and father blend together to form the traits observed in their offspring. As a result, traits are an intermediate between the mother’s and father’s.

X

? X ? ? X ? F1

F2 ? ? ? ?

Parental

Mendel’s Experiments with a Single Trait

• Mendel let the F1 generation mature and self-pollinate.

• Monohybrid cross – between two parents that each carry two different genetic determinants for the same trait (each F1 individual had a smooth and wrinkly parent)

• Outcome: 5474 round, 1850 wrinkly! (~3:1 ratio)

X

? X ? ? X ? F1

F2

Parental

Mendel’s Experiments with a Single Trait

• Outcome: 5474 round, 1850 wrinkly! (~3:1 ratio)

• Mendel coined these terms:

– Recessive – the wrinkled-seed shape seemed to recede or become hidden.

– Dominant – the round seeds appeared to dominate.

X

? X ? ? X ? F1

F2

Parental

? ? ? ?

Mendel’s Experiments with a Single Trait

• Repeated this experiment with other traits, had identical results, with a ratio of 3:1 in F2.

• Mendel formulated the Particulate Inheritance Hypothesis – Hereditary determinants maintain their integrity from generation to generation. Instead of blending together, they act as discrete entities or particles.

X

? X ? ? X ? F1

F2

Parental

? ? ? ?

Particulate Theory

• Inherited characters are determined by particular factors (now called genes).

• These factors occur in pairs (i.e., genes occur on maternal and paternal homologous chromosomes).

• When gametes form, these genes segregate so that only one of the homologous pair is contained in a particular gamete.

Mendel’s Laws • Mendel’s First Law, Law of Segregation – Members

of each gene pair must separate into different gamete cells during the formation of eggs and sperm. Each gamete has an equal chance of possessing either member of a pair of homologous chromosomes.

SS x ss S s Ss

Parental Genotype Parental Gametes F1 Genotype

Punnett Squares

• Monohybrid cross

• A = Yellow

• a = Green

• Genotype ratio: 1:2:1

• Phenotype ratio: 3:1

• Say we have a yellow pea plant that had a green pea plant as the mother. We cross the yellow pea plant to a green pea plant.

• A = Yellow • a = Green

• What is the genotype

of pea #2? • Of pea #3? • What possible gametes can

pea #2 produce?

Example (17.1)

x

x ? 1

2 3

17.2 - Determine color and height ratios for corn plants

• Examine ears of corn with

different color grains

• Record observations and determine the probable genotypes of the parents of each cross.

• Each kernel is a seed/individual.

• Cross involving two traits

Experiments with Two Traits

• Do different genes segregate together or independently?

• The Dihybrid Cross – Crossing two individuals heterozygous

for two traits.

• Possibilities: – Independent assortment: seed shape

and color will separate from each other and be transmitted independently.

– Allele for seed shape and seed color present always as seen in parent plants, meaning genes are transmitted together.

YyRr YyRr

x

YYRR yyrr Parental

F1

Mendel’s Laws

• Mendel’s Second Law, Law of Independent Assortment – Genes on nonhomologous or different chromosomes will be distributed randomly into gametes.

• Anaphase of Meiosis I

– Chromosomes segregate randomly

Codominance - Blood Type

• Phenotypes of both alleles expressed at the same time.

• Example: AB blood phenotype, IAIB genotype

• Today: Test blood for phenotype, determine genotype and possible paternity

Sex Linkage and Chromosome Theory • Sex-linked traits are located on

the sex chromosomes (X or Y)

XX - female XY - male

fruit fly

eye color

Polygenic Traits

• Trait influenced by more than one gene

• Examples: Blood type (e.g., O-); fur color in many species

Pleiotrophy

• One gene influences more than one trait

• Example: Sickle-cell anemia causes sickle-shaped red blood cells but also resistance to malaria

Environmental Influence on Gene Expression

• Phenotypes of most genes strongly influenced by the environment

• PKU (Phenylketonuria)

• Winter coats

Mendelian Genetics in Real Life

• Most traits are controlled by many genes, so Punnett squares may oversimplify

• Commonly used in selective breeding on simple traits

• May be used in genetics counselling

Today

• 17.1 – Determine genotypic and phenotypic ratios for albinism • 17.2 – Determine color ratios for corn plants – use from blue bin • Incomplete dominance • 17.3 – Determine blood type for ABO system • 17.4 – Determine Rh • Sex-linked inheritance - info only in real lab manual • 17.5 – Analyze a pedigree of inheritance of cystic fibrosis • 17.6 – Analyze a pedigree of inheritance of Huntington’s disease

• Answer questions on handout and turn in before leaving • Work in groups of 2-4, complete handout and turn in before

leaving. • Study for Quiz next week!