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Mendelian Genetics

Genetics Inheritance

IntroductionGenetics is the branch of biology that studies heredityGenetics is the branch of biology that studies the storage, duplication, and transfer of informationOrganisms inherit characteristics from their parentsThe information for these characteristics is contained in an organism’s DNA.

Gregor Mendel

The History of Modern GeneticsModern genetics began early in the 20th century with the pioneering work of Gregor MendelMendel worked on the garden pea (Pisum sativum), and he establishedthat certain traits could be passed from generation to generationPrior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution" of the different parental characteristics.

Gregor Mendel’s Peas

Gregor Mendel’s PeasMendel studied a number of characteristics in pea plants:

Plant height: short or tallSeed color: green or yellowSeed shape: wrinkled or roundFlower color: white or purplePod shape: constricted or smooth Pod color: yellow or green Flower position: terminal or axial


7 True-breeding phenotypes in pea


1. Self-fertilization2. Cross-pollination


Mendel’s Monohybrid CrossesMendel crossed tall and dwarf pea plants to investigate how height was inherited He emasculated the male plant then applied pollen the stigma of other varietyThe hybrids were uniformly tallThe same result for the reciprocal crossesThe dwarf seemed to disappear in the progenyMendel allowed them to undergo self-fertilization

among 1064 progeny: 787 tall and 277 dwarfratio of approximately 3:1

Mendel’s Monohybrid Crosses




Monohybrid Crosses Yielded Consistent Results

Mendel’s Monohybrid CrossesThe hybrids had the ability to produce dwarf progenyThe latent (masked) factor: recessive; the expressed factor: dominantThe cross was called a monohybrid crossEach trait was controlled by a heritable factor (genes)The dominant and recessive forms are called allelesEach of the parental strains carried two identical copies of a gene homozygousthe hybrid two different alleles heterozygous.

Mendel’s Monohybrid CrossesMendel used symbols to represent the hereditary factorsThe allele for dwarfness, (recessive): dThe allele for tallness (dominant): DThe tall and dwarf pea strains: DD and ddThe allelic constitution: genotypethe physical appearance (tall or dwarf): phenotypeAs the parental strains, the tall and dwarf pea plants form the P generation of the experiment.

Mendel’s Monohybrid CrossesThe hybrid progeny first filial generation (F1)The genotype F1: Dd (heterozygous)The F1 phenotype: tall because D is dominant over dDuring meiosis, these F1 produce two kinds of gametes, D and d, in equal proportions They separate, or segregate, from each other during gamete formationUpon self-fertilization, the two kinds of gametes produced by heterozygotes can unite in all possible ways.

Mendel’s Monohybrid CrossesThus, they produce four kinds of zygotes: DD, Dd, dD, and ddBecause of dominance, three of these genotypes have the same phenotypeThus, in the next generation, called the F2, the plants are either tall or dwarf, in a ratio of 3:1.

Genetics terms you need to know:Gene: a unit of heredity; a section of DNA sequence encoding a single proteinGenome: the entire set of genes in an organismAlleles: 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:Homozygous: having identical alleles (one from each parent) for a particular characteristicHeterozygous: having two different alleles for a particular characteristicDominant: the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous conditionRecessive: 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)

Genetics terms you need to know:Genotype: the genetic makeup of an organismsPhenotype: the physical appearance of an organism Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single traitP: Parental generationF1: First filial generation; offspring from a genetic crossF2: Second filial generation of a genetic cross.

(AA, Aa or aa)



Mendel’s Monohybrid CrossesMendel’s two principles of heredity:1. The Principle of Dominance:

In a heterozygote, one allele may conceal the presence of another

2. The Principle of Segregation: In a heterozygote, two different alleles segregate from each other during the formation of gametes.

Example 1In 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?

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?

Let B: black coat color; and b: brown coat

P: homozygous for black X homozygous brown coatBB bb

Gamete: B bF1: All Bb

Genotypic ratio: 100% BbPhenotypic ration: 100% Black coat

Example 2In 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?

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?

D: allele for polydactyly; d: alelle for normal fingerD is dominant over d

P: heterozygous polydactyly X normalDd dd

Gamete: D, d dF1: d

D Dd (polydactyly)d dd (normal)

Genotypic ratio: ½ Dd : ½ ddPhenotypic ratio: ½ polydactyly : ½ normal

Dihybrid CrossMendel investigated the inheritance of seed shape (smooth vs wrinkled) and seed color (green vs yellow) at the same timeFrom his monohybrid crosses he knew that smooth seeds were dominant to wrinkled seeds and yellow seeds were dominant to green seedsHe 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

Dihybrid CrossSeed color: 2 alleles g (green) and G (yellow), Seed texture: w (wrinkled) and W (round) The parental strains, doubly homozygous

yellow, round plants: GG WWgreen, wrinkled plants: gg ww

Gametes from GG WW plants: GWgametes from gg ww plants: gw Cross-fertilization produces F1 hybrids(doubly heterozygous) GgWwPhenotype yellow, round indicates that the G and W alleles are dominant.

Dihybrid CrossP: round, yellow x wrinkled, green

F1: 100% round, yellow seedsF1 intercross: round, yellow x round, yellow

F2 offspring: Total 556 seeds from F2 plantsRatio315 9 round and yellow seeds108 3 round and green seeds101 3 wrinkled and yellow seeds32 1 wrinkled and green seeds

Dihybrid Cross

Dihybrid Cross

Dihybrid Cross

Dihybrid CrossA dihybrid cross can be treated as two separate monohybrid crossesThe expected probability of each type of seed can be calculated:

Probability of an F2 seed being round = 75% or ¾Probability 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 CrossTherefore:

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

Mendel’s Principles3. Principle of Independent Assortment:The alleles of different genes segregate or assort independently of each other

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

Trihybrid crosses

P: AABBCC x aabbcc

Gamete: ABC abc

F1: AaBbCc

Gametes: ABC, ABc, AbC, Abc, aBC, aBc, abC, abc

Trihybrid Crosses

A

a

Types of gametes produced by AaBbCc

B

B

b

b cCcCcCcC

abcabCaBcaBCAbcAbCABcABC

Punnett Square for Trihybrid crosses♀ ♂ ABC ABc AbC Abc aBC aBc abC abc

ABC

ABc

AbC

Abc

aBC

aBc

abC

abc

AABBcc

AABbCc

AABbcc

AaBBCc

AaBBcc

AaBbCc

AaBbcc

AaBbCcAaBbCCAaBBCcAaBBCCAABbCcAABbCCAABBCcAABBCC

AAbBCc

AAbbCC

AAbbCc

AaBbCC

AaBbCc

AABBCC

AabbCc

AABbcc

AAbbCc

AAbbcc

AaBbCc

AaBbcc

AabbCc

Aabbcc

AaBBCc

AaBbCC

AaBbCc

aaBBCC

aaBBCc

aaBbCC

aaBbCc

AaBBcc

AaBbCc

AaBbcc

aaBBCc

aaBBcc

aaBbCc

aaBbcc

AaBbCc

AabbCC

AabbCc

aaBbCC

aaBbCc

aabbCC

aabbCc

AaBbcc

AabbCc

Aabbcc

aaBbCc

aaBbcc

aabbCc

aabbcc

AABBCc

AABbCC

AABbCc

AaBBCC

AaBBCc

AaBbCC

AaBbCc

Forked-line Method for Trihybrid CrossesSplit the individual geneMake crossesCombine them backGenotypes:

Aa x Aa ¼ AA : ½ Aa : ¼ aaBb x Bb ¼ BB : ½ Bb : ¼ bbCc x Cc ¼ CC : ½ Cc : ¼ cc

Phenotypes:Aa x Aa ¾ A- : ¼ aaBb x Bb ¾ B- : ¼ bbCc x Cc ¾ C- : ¼ cc

¼ x ¼ x ¼ = 1/64 AABBCC

¼ BB

¼ bb

½ Bb¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ CC¼ cc½ Cc

¼ BB

¼ bb

½ Bb

¼ BB

¼ bb

½ Bb¼ AA

¼ aa

½ Aa

¼ x ¼ x ½ = 2/64 AABBCc¼ x ¼ x ¼ = 1/64 AABBcc¼ x ½ x ¼ = 2/64 AABbCC¼ x ½ x ½ = 4/64 AABbCc¼ x ½ x ¼ = 2/64 AABbcc¼ x ¼ x ¼ = 1/64 AAbbCC¼ x ¼ x ½ = 2/64 AAbbCc¼ x ¼ x ¼ = 1/64 AAbbcc

¼ x ¼ x ¼ = 1/64 aaBBCC¼ x ¼ x ½ = 2/64 aaBBCc¼ x ¼ x ¼ = 1/64 aaBBcc¼ x ½ x ¼ = 2/64 aaBbCC¼ x ½ x ½ = 4/64 aaBbCc¼ x ½ x ¼ = 2/64 aaBbcc¼ x ¼ x ¼ = 1/64 aabbCC¼ x ¼ x ½ = 2/64 aabbCc¼ x ¼ x ¼ = 1/64 aabbcc

¼ x ¼ x ¼ = 1/64 AABBCC¼ x ¼ x ½ = 2/64 AABBCc¼ x ¼ x ¼ = 1/64 AABBcc¼ x ½ x ¼ = 2/64 AABbCC¼ x ½ x ½ = 4/64 AABbCc¼ x ½ x ¼ = 2/64 AABbcc¼ x ¼ x ¼ = 1/64 AAbbCC¼ x ¼ x ½ = 2/64 AAbbCc¼ x ¼ x ¼ = 1/64 AAbbcc

Trihybrid Cross: AaBbCc X AaBbCcG

enot

ypes

:

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

Phenotype

A-¾

aa¼

B-¾

bb¼

B-¾

bb¼

C-¾

cc¼

C-¾

cc¼

C-¾

cc¼

C-¾

cc¼ A-B-cc(¾) (¾) (¼)

A-bbC-(¾) (¾) (¼)

A-bbcc(¾) (¼) (¼) aaB-C-(¾) (¾) (¼)

aaB-cc(¾) (¼) (¼) aabbC-(¾) (¼) (¼)

aabbcc(¼) (¼) (¼)

A-B-C-(¾) (¾) (¾) A-B-cc9/64

A-bbC-9/64

A-bbcc9/64

aaB-C-3/64

aaB-cc3/64

aabbC-3/64

aabbcc1/64

A-B-C-27/64

Trihybrid Cross: AaBbCc X AaBbCc

How can you determine genotype from individual expressing dominant phenotype? -DD or Dd?Cross individual with dominant phenotype to a homozygous recessive individual.

Monohybrid Test Cross

Dihybrid test crossIn monohybrid crosses, to know if a dominant trait is homozygous (DD) or heterozygous (Dd) it is necessary to carry out a test crossThis is done with a homozygous recessive (dd) individualThe same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (wwgg)

Dihybrid testcrossTestcross results of four smooth round individualsWWGG

x s x

wg

WG

Phenotypic ratioAll smooth yellow

WWGg

x

wg

WG Wg

Phenotypic ratio½ smooth yellow½ smooth green

WwGg

x

wg

WGWgwG

wg

Phenotypic ratio¼ smooth yellow¼ smooth green¼ wrinkle yellow¼ wrinkle green

WwGG

x

wg

WG wG

Phenotypic ratio½ smooth yellow½ wrinkle yellow

WwGg

Wwgg

WwGg

wwGgWwGg WwGg

Wwgg

wwGg

wwgg

WWGG wwgg wwggWwGG WWGg WwGgwwgg wwgg

All smooth yellow All smooth MixedAll yellow

Monohybrid and Dihybrid Problems1. 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?

2. 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?

Monohybrid and Dihybrid ProblemsTodd 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?

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?

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 and Dihybrid Problems

7. 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?

8. 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?

Monohybrid and Dihybrid Problems

9. 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