Mendelian Genetics

Genetics Inheritance

Introduction

۞ Genetics is the branch of biology that studies heredity

۞ Genetics is the branch of biology that studies the storage, duplication, and transfer of information

۞ Organisms inherit characteristics from their parents

۞ The information for these characteristics is contained in an organism’s DNA.

Gregor Mendel

The History of Modern Genetics

۞ Prior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution"of the different parental characteristics.

۞ Modern genetics began early in the 20th century with the pioneering work of Gregor Mendel

۞ Mendel worked on the garden pea (Pisum sativum), and he established that certain traits could be passed from generation to generation

Gregor Mendel’s Peas

۞ Mendel studied a number of characteristics in pea plants:

Plant height - short or tall

Seed color - green or yellow

Seed shape - wrinkled or round

Flower color - white or purple

Pod shape - constricted or smooth

Pod color - yellow or green

Flower position - terminal or axial

Gregor Mendel’s Peas

7 True-breeding phenotypes in pea

Gregor Mendel’s Peas

Gregor Mendel’s Peas

Cross Pollinating Peas

۞ Instead of self pollination, the pollen from one plant is used to pollinate the egg cells of another plant

۞ Cut away anthers

۞ Hand pollinated using pollen from a chosen plant.

Gregor Mendel’s Peas

1. Self-fertilization 2. Cross-pollination

Gregor Mendel’s Peas

۞ Gene – a unit of heredity; a section of DNA sequence encoding a single protein

۞ Genome – the entire set of genes in an organism

۞ Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (Different forms of a specific gene) (A or a)

۞ Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.

Genetics terms you need to know:

Genetics terms you need to know:

۞ Homozygous – having identical alleles (one from each parent) for a particular characteristic

۞ Heterozygous – having two different alleles for a particular characteristic

۞ Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition

۞ Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous.

(AA or aa)

(Aa)

(A)

(a)

۞ Genotype – the genetic makeup of an organisms

۞ Phenotype – the physical appearance of an organism (Genotype + environment)

۞ Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait

۞ P = Parental generation

۞ F1 = First filial generation; offspring from a genetic cross

۞ F2 = Second filial generation of a genetic cross.

(AA, Aa or aa)

Genetics terms you need to know:

round wrinkled

X

All round

F1:

selfing

round wrinkled

F2:

5474 : 1850

2.96 : 1

Mendel's data: Monohybrid Cross

Mendel's insight

"round" F1 is actually heterozygous: Aa A = round - dominant a = wrinkled - recessive Parents are AA and aa

P:

X

F1

selfing F2

705 : 224

3.15 : 1

Mendel's insight

“purple" F1 is actually heterozygous: Bb B = purple - dominant b = white - recessive Parents are BB and bb

Law of segregation: alleles segregate from each other in gametes, 1/2 get one, 1/2 get the other.

Mendel's data: Monohybrid Cross

P

purple parent

all P sperm and eggs

pp

p

white parent

all p sperm and eggs

p

PP P P

Gamete Formation: Parents

purple F1

P and p in sperm and eggs

Pp

P

P

Gamete Formation: F1

p

p

Determining the outcome of a single-trait cross

Phenotypic ratio:

3 purple:1white

Genotypic ratio:

1 PP : 2 Pp : 1pp

Use of Punnet Square for F2

♂ ♀

PP Pp

Pp pp

P p

P

p

Monohybrid Cross

P

P

P

p

Homozygous

Heterozygous

Therefore, the Principle of Segregation indeed is a general principle of genetics.

Monohybrid Crosses Yielded Consistent Results

Consistency is Good

No matter what the character, Mendel observed a 3:1 ratio of characters in the F2.

۞ Mendel investigated the inheritance of seed shape (smooth v wrinkled) and seed color (green v yellow) at the same time

۞ From his monohybrid crosses he knew that smooth seeds were dominant to wrinkled seeds and yellow seeds were dominant to green seeds

۞ He chose to cross plants that were pure breeding for both dominant features (smooth and yellow seeds) with plants that were pure breeding for both recessive features (wrinkled and green seeds).

Dihybrid Cross

Parents plants with round x plants with wrinkled and yellow seeds and green seeds

F1 offspring 100% plants with round and yellow seeds

F1 intercross plants with round x plants with round

and yellow seeds and yellow seeds

F2 offspring Total 556 seeds collected from F2 plants

Ratio

315 9 round and yellow seeds

108 3 round and green seeds

101 3 wrinkled and yellow seeds

32 1 wrinkled and green seeds

However, the ratio of dominant to recessive features is still 3:1

Dihybrid Cross

Law of independent assortment:

Alleles of separate genes assort

independently into the gametes

Genotypic ratio =

1SSYY : 2SSYy : 1SSyy : 2SsYY : 4SsYy : 2Ssyy : 1ssYY : 2ssYy : 1ssyy

Phenoypic ratio =

9 : 3 : 3 : 1

SY

Sy

sY

sy

Dihybrid Cross

۞ A dihybrid cross can be treated as two separate monohybrid crosses

۞ The expected probability of each type of seed can be calculated: Probability of an F2 seed being round = 75% or ¾

Probability of an F2 seed being wrinkled =25% or ¼

Probability of an F2 seed being yellow = 75% or ¾

Probability of an F2 seed being green = 25% or ¼

Dihybrid Cross

Therefore:

Probability of an F2 seed being round and yellow

= ¾ x ¾ = 9/16 = 56.25%

Probability of an F2 seed being round and green

= ¾ x ¼ = 3/16 = 18.75%

Probability of an F2 seed being wrinkled and yellow

= ¼ x ¾ = 3/16 = 18.75%

Probability of an F2 seed being wrinkled and green

= ¼ x ¼ = 1/16 = 6.25%

Dihybrid Cross

Phenotype We could expect

What Mendel observed

556 x 9/16 round yellow 313 315

556 x 3/16 round green 104 108

556 x 3/16 wrinkled yellow 104 101

556 x 1/16 wrinkled green 35 32

Dihybrid Cross

Trihybrid crosses

P: AABBCC x aabbcc

Gamete: ABC abc

F1: AaBbCc

Gametes: ABC, ABc, AbC, Abc,

aBC, aBc, abC, abc

Trihybrid Crosses

A

a

AaBbCc

B

B

b

b c C c C

c C

c

C

abc

abC aBc

aBC Abc AbC

ABc

ABC

Trihybrid Cross - Phenotypes

Phenotypic ratio: 27:9:9:9:3:3:3:1

Forked-line Method

-

-

-

-

-

-

- bb

bb

aa

cc

cc

cc

cc

aabbC-

aabbcc

aaB-cc

aaB-C-

A-bbcc

A-bbC-

A-B-cc

A-B-C-

aabbC-

aabbcc

aaB-cc

aaB-C-

A-bbcc

A-bbC-

A-B-cc

A-B-C-

Mendel’s Principles

۞ Principle of Dominance:

One allele masked another, one allele was dominant over the other in the F1 generation

۞ Principle of Segregation:

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 (allele).

۞ Principle of Independent Assortment:

“Members of one gene pair segregate independently from other gene pairs during gamete formation”

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

Mendel’s Principles

Exercises

1. For the following genotypes, tell how many gametes will be produced

a) BB

b) Ss

c) DDRr

Solution

Use this formula:

where n is number of heterozygous genes.

a) BB, n=0, 20=1 gamete: B

b) Ss, n=1, 21=2 gametes: S, s

c) DDRr, n=1, 21=2 gametes: DR, Dr

# gametes = 2n

Exercises

2. List all possible gametes produced by

a) aabbcc

b) kkLLMm

c) PpQQRr

d) RrSsTt

Solution:

a) aabbcc: 1 gametes: abc

b) kkLLMm: 2 gametes: kLM and kLm

c) PpQQRr: 4 gametes: PQR, PQr, pQR, pQr

d) RrSsTt: 8 gametes: RST, RSt, RsT, Rst,

rST, rSt, rsT, rst

Monohybrid Test Cross

How can you determine genotype from individual expressing dominant phenotype? - DD or Dd?

Cross individual with dominant phenotype to a homozygous recessive individual.

Dihybrid test cross

۞ In monohybrid crosses, to know if a dominant trait is homozygous (SS) or heterozygous (Ss) it is necessary to carry out a test cross

۞ This is done with a homozygous recessive (ss) individual

۞ The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (ssyy)

Dihybrid testcross

Testcross results of four smooth round individuals

SSYY x s x sy

Sy Phenotypic ratio All smooth yellow

SSYy x sy

SY Sy Phenotypic ratio ½ smooth yellow ½ smooth green

SsYy x sy

SY

Sy

sY

sy Phenotypic ratio ¼ smooth yellow ¼ smooth green ¼ wrinkle yellow ¼ wrinkle green

SsYY x sy

SY sY Phenotypic ratio

½ smooth yellow

½ wrinkle yellow

SsYy

Ssyy

SSYy

ssYy

SsYy SsYy

Ssyy

ssYy

ssyy

SSYY SsYY SSYy SsYy

SSYY ssyy ssyy SsYY SSYy SsYy ssyy ssyy

All smooth yellow All smooth Mixed All yellow

Monohybrid Problems

1. In rabbits the allele for black coat color (B) is dominant over the allele for brown coat color (b). What is the genotypic ratio and phenotypic ratio be for a cross between an animal homozygous for black coat color and one homozygous for brown coat color?

2. White (W) hair in sheep is caused by the dominant gene while black (w) hair is recessive. A heterozygous white male and a black female are parents of a black lamb. What is the probability that their next lamb will be white? What are the genotypic and phenotypic ratios?

3. Albinism is recessive in humans. An albino man marries a woman who is not albino, but had an albino father. What is the probability of this couple having a child that is not an albino? What are the genotypic and phenotypic ratios?

4. Todd and Melissa are college students who are planning to get married. They are currently taking a genetics course and decided to determine the eye color of any possible children they might have. Blue eyes are recessive to brown eyes. Todd has brown eyes, like his three brothers. His mother and grandmother have blue eyes, but his father and all other grandparents have brown eyes. Brown eyed Melissa has one blue eyed sister and one brown eyed sister and a mother with blue eyes. Her father and all of her grandparents have brown eyes. Construct an accurate punnett square to determine the possible eye colors of their yet to be born children. What are the genotypic and phenotypic ratios?

5. In Teenage Mutant Ninja Turtles, green shells are dominant over brown shells. Leonardo, who is heterozygous for a green shell, marries the lovely Mona Lisa, who has a brown shell. What are the genotypic and phenotypic ratios?

6. In humans, polydactyly (an extra finger on each hand or toe on each foot) is due to a dominant gene. When one parent is polydactylous, but heterozygous, and the other parent is normal, what are the genotypic and phenotypic ratios of their children?

Monohybrid Problems

5. In guinea pigs, rough coat (R) is dominant over smooth coat (r). A rough coated guinea pig is bred to a smooth one, giving eight rough and seven smooth progeny in the F1.

a) What are the genotypes of the parents and their offspring?

b) If one of the rough F1 animals is mated to its rough parent, what progeny would you expect?

6. In summer squash, white fruit (W) is dominant over yellow (w), and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d). The following problems give the phenotype of the parents and their offspring. Determine the genotypes of the parents in each case:

a) White, disk x yellow, sphere gives 1/2 white, disk and 1/2 white, sphere.

b) White, sphere x white, sphere gives 3/4 white, sphere and 1/4 yellow, sphere.

c) Yellow, disk x white, sphere gives all white, disk progeny.

Monohybrid Problems

Dihybrid Problems

1. In human, aniridia, (a type of blindness resulting from absence of an iris) is due to a dominant gene. Migraine (a sickening headache) is due to a different dominant gene. A man with aniridia, whose mother was not blind, marries a woman who suffers from migraine. The woman’s father did not suffer from migraine. In what proportion of their children would both aniridia and migraine be expected to occur?

2. In watermelons, solid green color (G) is dominant over striped pattern (g), and short shape (S) is dominant over long shape (s). What is the probability of each phenotype of possible offspring if a heterozygous solid, long watermelon cross pollinates with a heterozygous solid, heterozygous short watermelon?

3. Having two eyebrows is dominant (E) over having one large eyebrow (e). Also having six fingers (F) is dominant over having five fingers (f). What is the probability of each phenotype if a man that has one eyebrow and twelve fingers total (heterozygous), has children with a woman that is heterozygous for both traits?

Any Questions? Thank You