Genetic Inheritance Problems - Exercise 9 Objectives -Know how to apply basic genetic terms. -Know how to do compute Punnett squares of monohybrid and.

Genetic Inheritance Problems - Exercise 9Objectives

-Know how to apply basic genetic terms.-Know how to do compute Punnett squares of monohybrid and dihybrid crosses. -Know how to do sex-linked crosses. -Be able to apply Incomplete Dominance and Codominance.

Genetic Inheritance Problems

Genetics is the study of the hereditary material of life. The hereditary material (known as genes) is encodedas molecules of DNA on chromosomes. Genes canalso be symbolized as letters, called alleles. Allelesare alternate forms of genes found at a particularsport on a chromosome. The place where a gene orallele is found is called the locus.

Hereditary material (genes) in DNA on chromosomes.

In diploid animals, alleles exist in pairs.Before alleles and chromosomes are passedfrom parents (P1 generation) to offspring (F1

generation), the allelic and chromosomalpairs are separated by the process of meiosis. The result of meiosis in animals is theproduction of haploid gametes – egg andsperm. The alleles of the haploid gametesare then combined during reutilization toproduce the diploid offspring (zygote).

Basic Terms in Genetics

Alleles

• Dominant - expressed when paired with a different allele– Represented by an uppercase letter (RR) or (Rr)

• Recessive - no effect when paired with a dominant allele– Represented by a lowercase letter (rr)

Alleles in an Individual• Homozygote - same two alleles (AA or aa)• Heterozygote - two different alleles (Aa)• Genotype - the genetic makeup (Is its genetic

makeup)• Phenotype - observable characteristics (Is its physical

appearance)

Useful Genetic Vocabulary

• An organism that is homozygous for a particular gene– Has a pair of identical alleles for that gene– Exhibits true-breeding

• An organism that is heterozygous for a particular gene– Has a pair of alleles that are different for that gene

Genetic Cross• When two individuals are mated• P generation - parent generation• F1 generation - first generation

• F2 generation - second generation

• The hybrid offspring of the P generation– Are called the F1 generation

• When F1 individuals self-pollinate– The F2 generation is produced

• In a typical breeding experiment– Mendel mated two contrasting, true-breeding

varieties, a process called hybridization

• The true-breeding parents– Are called the P generation

Character - a heritable feature, such as flower color

Trait - a variant of a character, such as purple or whiteflowers

Extensions of Mendel’s Laws

• Many alleles do not show complete dominance– Incomplete dominance– Codominance– Epistasis– Environmental effects– Polygenic traits

• Incomplete Dominance: One does not completely cover the other. Halfway between two extremes, so blending of one another.

-Classic example: A red and white flower is cross, so you end up with a pink flower.

• Codominance: There are equally strong, so nothing over powers the other. Equal in strength, you see both phenotypes.

-Classic example: Cow having brown and white spots (Roam).

Incomplete Dominance• Heterozygote is an

intermediate• Horses• Snapdragons

P Generation

F1 Generation

F2 Generation

RedCRCR

Gametes CR CW

WhiteCWCW

PinkCRCW

Sperm

CR

CR

CR

Cw

CR

CRGametes1⁄2 1⁄2

1⁄2

1⁄2

1⁄2

Eggs1⁄2

CR CR CR CW

CW CWCR CW

The phenotype of F1 hybrids is

somewhere between the phenotypesof the two parental varieties

INCOMPLETE DOMINANCE

• Characterized by an absence of complete dominance in one allele.

• This manifests as a “blending” of traits, or a “hybrid” phenotype.

• Common in flower color genes.

4 O’ CLOCKS

red flowers X white flowers RR X rr [ R1R1] [R2R2]

Rr [R1R2] F1

pink

¼ RR [R1R1] red

½ Rr [R1R2] pink

¼ rr [R2R2] white

1:2:1 genotypic1:2:1 phenotypicWhen 2 heterozygotesare crossed

IDENTIFYING CHARACTERISTICS OF INCOMPLETE DOMINANCE

1. Traits are blended.2. Crossing two heterozygous individuals in a monohybrid

cross produces a 1:2:1 genotypic ratio and 1:2:1 phenotypic ratio.

Incomplete dominance is an apparent exception to Mendel’sFirst Law because a different phenotypic ratio is obtained.

The alleles are in fact segregating according to Mendel’s firstlaw, the mechanism by which the phenotype is produced isdifferent than in pea plants.

Codominance

• In codominance– Two dominant alleles affect the phenotype in

separate, distinguishable ways• The human blood group MN

– Is an example of codominance• Also ABO blood groups

CODOMINANCE

• A codominant gene in a heterozygous individual will express the phenotype of both alleles. The phenotype of both alleles are expressed independently.

• ABO blood groups in humans are an example. The I gene (isoagglutinogen) has three alleles (A, B, and O).

• The A and B alleles are dominant to O and codominant to each other.

ABO GENOTYPES

PHENOTYPE GENOTYPE ANTIGEN ANTIBODY A IAIA or IAIO A ANTI-B

B IBIB or IBIO B ANTI-A

O IOIO NONE BOTH

AB IAIB A and B NONE

BLOOD TYPE A

A ANTIGEN

BLOOD TYPE B

B ANTIGEN

BLOOD TYPE AB

BOTH A +B ANTIGENS

BLOOD TYPE O

NO ANTIGENS

Codominance• Both alleles are expressed• Seen in blood types

– IAIA or IAi = type A– IBIB or IBi = type B– ii = type O– IAIB = type AB

ABO Blood Groups

The ABO blood group in humans Is determined by multiple alleles.

GENES

• Genes are discrete units of heredity determining biological characteristics of living things.

• Genes exist in pairs in diploid organisms.• Alleles are alternate forms of the same gene,

each is on a different homologous chromosome.

GENOTYPE AND PHENOTYPE

• The genotype is the genetic constitution of the individual, in other words, the genes (and alternate forms) that are carried.

• Alternate forms of the same gene are called alleles.

• The phenotype is the observable trait (characteristic) produced by the genotype (gene).

MENDEL’S FIRST LAWSEGREGATION

• Governs the behavior of alleles.• 3 important observations from Mendel’s

crosses.• [1] Hybridization between two traits showed

only one trait in the offspring. white flowers X purple flowers

purple flowers

Basic Patterns of Inheritance

• Mendel started with true breeding plants

• Recessive trait skipped a generation

Mendel’s First Law: Law of Segregation

Phenotype versus Genotype

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

yellow seeds X green seeds [parental generation]

YY yy [P1]

yellow seeds [first filial generation]

Yy [F1]

¾ yellow seeds [F2]

¼ green seeds [second filial generation]

GENERATION DESIGNATIONS

• The parental generation (P1) is the first generation of the controlled cross.

• The first filial generation (F1) is the result of crossing the parental generation.

• The second filial generation (F2) is produced from the crossing of the F1 progeny.

PUNNETT SQUARE

• Graphical means of visualizing a monohybrid cross and applying probability to the outcome.

• E.G. cross 2 heterozygous individuals [Yy]

Y y

Y

y

YY Yy

Yy yyyellow yellow

yellow green

GenotypicRatio=1:2:1¼ YY½ Yy¼ yy

PhenotypicRatio=3:1¾ yellow seeds¼ green seeds

MENDEL’S SECOND LAWINDEPENDENT ASSORTMENT

• Governs the behavior of different genes.• Mendel started with two hypotheses.[1] All traits from 1 parent would be

transmitted together and only two types of offspring would result.

[2] Traits would be inherited independently and there would be more than two types of offspring.

• Using the information from a dihybrid cross, Mendel developed the law of independent assortment– Each pair of alleles segregates independently

during gamete formation

• Mendel identified his second law of inheritance– By following two characters at the same time

• Crossing two, true-breeding parents differing in two characters– Produces dihybrids in the F1 generation,

heterozygous for both characters

Independent assortment

Mendel’s Second Law: Law of Independent Assortment

Characteristics Studied

YYRRP Generation

Gametes YR yr

yyrr

YyRrHypothesis ofdependentassortment

Hypothesis ofindependent

assortment

F2 Generation(predictedoffspring)

1⁄2 YR

YR

yr

1 ⁄2

1 ⁄2

1⁄2 yr

YYRR YyRr

yyrrYyRr

3 ⁄41 ⁄4

Sperm

Eggs

Phenotypic ratio 3:1

YR1 ⁄4

Yr1 ⁄4

yR1 ⁄4

yr1 ⁄4

9 ⁄163 ⁄16

3 ⁄161 ⁄16

YYRR YYRr YyRR YyRr

YyrrYyRrYYrrYYrr

YyRR YyRr yyRR yyRr

yyrryyRrYyrrYyRr

Phenotypic ratio 9:3:3:1

315 108 101 32 Phenotypic ratio approximately 9:3:3:1

F1 Generation

Eggs

YR Yr yR yr1 ⁄41 ⁄4

1 ⁄41 ⁄4

SpermRESULTS

CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.

EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.

Dihybrid Cross – two characters

DIHYBRID CROSS WITH GENOTYPES

• A cross involving two traits.

round,yellow seeds X wrinkled, green

All round, yellow [F1]

R=roundr=wrinkled

Y=yellowy=green

RRYY rryy

RrYy

MENDEL’S EXPERIMENT THE DIHYBRID CROSS

• The dihybrid cross, a cross involving two traits. round,yellow seeds X wrinkled, green

All round, yellow [F1]

9/16 round, yellow

3/16 wrinkled, yellow 3/16 round, green 1/16 wrinkled, green

Phenotypic ratio=9:3:3:1

• Many genetic disorders– Are inherited in a recessive manner

• Recessively inherited disorders– Show up only in individuals homozygous for the

allele• Carriers

– Are heterozygous individuals who carry the recessive allele but are phenotypically normal

Recessively Inherited Disorders

Inheritance of Sex-Linked Genes• The sex chromosomes

– Have genes for many characters unrelated to sex

• A gene located on either sex chromosome– Is called a sex-linked gene

Other sex-linked conditions• Some recessive alleles found on the X chromosome in

humans cause certain types of disorders– Color blindness– Duchenne muscular dystrophy– Hemophilia

Sex-linked genes follow specific patterns of inheritance

Figure 15.10a–c

XAXA XaY

Xa Y

XAXa XAY

XAYXAYa

XA

XA

Ova

Sperm

XAXa XAY

Ova XA

Xa

XAXA XAY

XaYXaYA

XA YSperm

XAXa XaY

Ova

Xa Y

XAXa XAY

XaYXaYa

XA

Xa

A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.

If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.

If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele.

(a)

(b)

(c)

Sperm

Figure 15.10a–c

XAXA XaY

Xa Y

XAXa XAY

XAYXAYa

XA

XA

Ova

Sperm

XAXa XAY

Ova XA

Xa

XAXA XAY

XaYXaYA

XA YSperm

XAXa XaY

Ova

Xa Y

XAXa XAY

XaYXaYa

XA

Xa

A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.

If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.

If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele.

(a)

(b)

(c)

Sperm

Questions - Page 7 - Lab Book • 1. State Mendel’s First Law. What part of meiosis is

the basis for this law? • 2. State Mendel’s Second Law. What part of the

meiotic process is the basis for this law?

• 3. Why do we use a Punnett squares to solve genetic problems?

-Can Punnett squares give us precise outcomes of anoffspring?

Questions - Page 7 - Lab Book

• 1. State Mendel’s First Law. What part of meiosis is the basis for this law?

Two alleles from a heritable character separate during gamete formation andend up in different gametes, during Anaphase I. • 2. State Mendel’s Second Law. What part of the meiotic process is the basis for this

law? Each pair of alleles segregates independently of other pairs of alleles duringgamete formation, during metaphase I.

• 3. Why do we use a Punnett squares to solve genetic problems? Shows all the possibilities of the combination of alleles in an offspring thatresults from a cross whether its monohybrid or dihybrid. • -Can Punnett squares give us precise outcomes of an offspring? No, it gives you

possibilities of what could happen not actual outcomes.

If an allele for tall plants (T) is dominant to short plants (t), what offspring would youexpect from a TT x Tt cross?

A. ½ tall; ½ shortB. ¾ tall; ¼ shortC. All tall

Questions

If an allele for tall plants (T) is dominant to short plants (t), what offspring would youexpect from a TT x Tt cross?

A. ½ tall; ½ shortB. ¾ tall; ¼ shortC. All tall

Questions

Fur color in rabbits shows incomplete dominance.FBFB individuals are brown, FBFW individuals arecream, FWFW individuals are white. What is theexpected ratio of a FBFW x FWFW cross?

A. 3 white: 1 brownB. 3 white: 1 creamC. 2 white: 2 cream

Questions

Fur color in rabbits shows incomplete dominance.FBFB individuals are brown, FBFW individuals arecream, FWFW individuals are white. What is theexpected ratio of a FBFW x FWFW cross?

A. 3 white: 1 brownB. 3 white: 1 creamC. 2 white: 2 cream

Questions

• Height in pea plants is determined by the genes T (dominant) and t (recessive).

• Cross a homozygous tall pea plant with a dwarf pea plant and determine the probability of producing a tall plant.

Questions - Monohybrid Cross

• Height in pea plants is determined by the genes T and t.

• Cross two heterozygous tall plants and determine the probability of producing a dwarf plant.

Questions - Monohybrid Cross

• Note that blood type genotypes may be written using an "I" before the A and B, such as IAIA and IBi, etc. In this problem I’m not using "I".

• Hazel has type B blood (genotype BO) and Elijah has type O blood (genotype OO). If they have children, what is the probability that they will have a type B child? What is the probability they will have a type A child?

• In this problem you are given the genotypes so you know both genes for each blood type.

Questions

• When a genetic cross involves the consideration of two factors (such as shape and colour in pea seeds), the cross is called a "dihybrid".

• Cross a completely heterozygous round/yellow seeded plant with a completely homozygous round/green seeded plant.

• Then determine the probability of obtaining a round/yellow seeded plant in the offspring.

• R = round seeds, r = wrinkled seedsY = yellow seeds, y = green seeds

Questions - Dihybrid Cross

Questions - Page 7 - Lab Book

Questions - Page 7 - Lab Book

Questions - Page 7 - Lab Book

Questions - Page 7 - Lab Book