MENDELIAN GENETICS

OBJECTIVES• Understand Mendel’s principles governing genetics

• Understand meaning of relevant vocabulary

• Predict results of mono/dihybrid crosses using Punnett square

• Know exceptions to rules

GENETICS• Genetics:

• Heredity:

GENETICS TERMINOLOGY

Gene (Character):

Locus (loci= pl.):

Allele (Trait):

Genotype:

Phenotype:

MENDEL• Gregor Mendel

– Interested in basic patterns governing the transmission of traits from parent to offspring

– Mendel’s work precedes an understanding of meiosis/genes/chromosomes

HISTORICAL HYPOTHESES REGARDING INHERITANCE

Blending Inheritance Hypothesis:

Inheritance of Acquired Characteristics Hypothesis

MENDEL• Mendel performed crosses between pure bred

garden peas• Began studying inheritance of ONE trait

• Later, multiple traits studied together

MENDEL’S CROSSES• Mendel's plants had two observable “forms”

(alleles) of each “character” (gene)

– Green or Yellow seeds • Round or Wrinkled seeds

Trait Forms of Trait (Phenotype)

Seed shapeRound Wrinkled

Seed color

Pod shape

Pod color

Yellow Green

Inflated

Green

Constructed

Yellow

Round Wrinkled

Yellow Green

Smooth Constricted

Green Yellow

or

or

or

or

Trait Forms of Trait (Phenotype)

Flower color

Purple White

Axial (on stem) Terminal (at tip)

Flower andpod position

Stem length

Tall Dwarf

Purple White

Axial (on stem) Terminal (at tip)

Dwarf Tall

or

or

or

MENDEL

• Looked at traits exhibited by progeny of crosses

– Found observable evidence of how parents transmit traits to offspring

MENDEL’S PEA PLANTS

• Pisum sativum

Stigma (receives pollen)

Anthers(produce pollengrains, whichcontain malegametes)

Ovules (produce female gametes)

Male parts = anthers–Pollen grain contain sperm

Female parts = pistil–Ovule = eggs

SELF FERTILIZATION

MENDEL’S PEA PLANTSControlled Mating:

– Cut off reproductive organ to stop self fertilization

– Cross fertilize plants in a controlled fashion– Plant that contributes pollen

– Plant that receives the pollen

CROSS-POLLINATIONCROSS-POLLINATION

1. Remove anthersfrom one plant.

2. Collect pollenfrom a different plant.

3. Transfer pollento stigma of the 1st plant(plant without anthers)

MENDEL’S CROSSESRound Parent Wrinkled Parentx

MENDEL’S CROSSESNext, Mendel crossed F1 (bro-sis mating):

Round (F1) Round x

MENDEL’S CROSSESMendel Determines (Phenotypic) Ratio:

F2: 5474 Round : 1850 Wrinkled

– Similar pattern observed in other 6 traits

MENDEL’S CONCLUSIONS

MENDEL’S SYMBOLS• Mendel assigned symbols to hereditary

characters

• Letters represent versions of a gene (alleles):– Ex: If Letter “r” represents a gene then:

PUNNETT SQUARE• R.C. Punnett

– Leading geneticist in early 1900s

– Invented technique called the “Punnett Square”– Place gamete possibilities for 2 parents along axes

– Internal boxes represent union of gametes

– Used to predict genotype and phenotype of potential zygote

MENDEL’S CROSS OF PURE LINE PEA PLANTS

PARENT: R RP

AR

EN

T:

r r

** hint: gametes along top, “whole people” on inside

R is roundr is wrinkled

MENDEL’S CROSS OF F1 PLANTS

• Phenotypic Ratio: • Genotypic Ratio

Crossing the F1s: Rr x Rr

R = roundr = wrinkled

POSSIBLE GENOTYPESThree possible genotypes for each gene in a diploid cell:• Homozygous:

• Homozygous Dominant:

• Homozygous Recessive:

• Heterozygous:

PROBABILITY THEORYIn Diploid Organisms,

• Each allele has 50% chance of being found in a particular gamete

• To calculate likelihood of two alleles combining (i.e. fertilization):

MENDEL’S CROSSES• Dihybrid Crosses:

– Mendel used pea plants to follow inheritance of TWO traits

• Another pattern began to emerge…

MENDEL’S SYMBOLS

• Seed Shape:• R is dominant allele (Round)

• r is recessive allele (Wrinkled)

• Seed Color:• Y is dominant allele (Yellow)

• y is recessive allele (Green)

F2 generationphenotype

Mendel’sNumbers

315 101 108 32 = 556

Fraction ofprogeny

9/16 3/16 3/16 1/16 = 1

F2 OFFSPRING FROM DIHYBRID CROSS

Note: New mixtures of traits are observed in F2!!

PUNNETT SQUARE FOR TWO TRAITSDo genes segregate into gametes

together or independently?

PUNNETT SQUARE FOR TWO TRAITS

If genes segregate independently

of each other, we end up with

MENDEL’S LAW OF INDEPENDENT ASSORTMENT

• Law of Independent Assortment:

INDEPENDENT ASSORTMENT

OR

During meiosis I, homologs can line up two different ways before separating.

Brown eyes

Black hairRed hair

Blue eyes

THE TEST CROSS

• To determine genotype of an individual

MENDELWhy Mendel Was Successful:

• Unbiased• Selected good model organism• Used pure breeds as parents• Large sample size• Quantitative analysis• Controlled experiments• Studied obvious traits

CONCLUSIONS: MENDELIAN GENETICS