Dihybrid Crosses - looking at 2 traits

Dihybrid Crosses - looking at 2 traits

Mendel’s dihybrid crosses:

1. Mendel also performed crosses involving two pairs of traits, e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green).

2. If alleles sort independently, four possible phenotypes (2n) appear in the F2 generation in a 9:3:3:1 ratio.

“Mendel’s Principle of Independent Assortment”:

Alleles for different traits assort independently of one another.

Modern formulation of independent assortment:

Genes on different chromosomes behave independently in gamete production.

The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment

The alignment of one pair of homologs is independent of any other.Principle of Independent Assortment: The

assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

Possible Gametes:To determine the number of different gametes a parent can have use the FOIL method.

RREE

RE RE RE RE

All gametes are RE


RREe

Re RE RE Re

You have 2 possible gametes: Re and RE


rrEe

re rE rE re

You have 2 possible gametes: rE and re

How many gametes?To determine the number of different gametes a parent can have use the FOIL method.

TTYY =

Number and kind of gametes

one TY

TtYY

TtYy

=

=

two TY and ty

four TY, Ty, tY, ty

If I cross:

What are the different possible gametes for these parents??

RRTT x RrTt

Possible Gametes (sperm/egg):

RT RT, Rt, rT, rt

There are 4 total different gametes

If I cross:What are the possible gametes?

RRTT x RrTt

RT

rtrTRtRT

RRTT RrTtRrTTRRTt

I have 4 possible offspring

If I cross:What are the possible gametes and offspring?

rrTt x RRTt

rT

RtRT

RrTT RrTt


rt RrTt Rrtt

If I cross:What are the possible gametes?

RrTt x RrTt

RT

rtrTRtRT

RRTT RrTtRrTTRRTt


9:3:3:1

Rt

rT

rt

RRTt RRtt RrTt Rrtt

RrTT RrTt rrTT rrTt

RrTt Rrtt rrTt rrtt

Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another?

Because it’s the only way to explain

the pattern of inheritance!

Phenotype Ratio:9 yellow/smooth3 yellow/wrinkled3 green/smooth1 green/wrinkled

Consider a cross between parents heterozygous for both deafness and albinism.

This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.

What Works for Peas Also Works for Humans

January 10, 2012 Get your homework out on your desk!

Warm Up #1: A pea plant with the genotype TtWW is

crossed with a pea plant with the genotype ttWw. How many different genotypes can be expressed in the offspring?

(SHOW ALL WORK!)a. 1 b. 2 c. 3 d. 4

Warm Up #2

If a corn plant has a genotype of Ttyy, what are the possible genetic combinations that could be present in a single grain of pollen (sperm, male gamete) from this plant?

a. Ty, tyb. TY, tyc. TY, Ty, tyd. Ty, ty, tY, TY

Warm Up #3

3. The pea plant produces plants of two different sizes and seeds that are two different shapes. Tall pea plants (T) are dominant to dwarf pea plants (t) and round seeds (R) are dominant to wrinkled seeds, r. Two plants heterozygous for both traits are mated. What fraction of their offspring will be dwarf and have wrinkled seeds?

(SHOW ALL WORK!)

Great job on your quizzes! Hand back quizzes and work – review

most common mistakes.

Tonight: Talk to your parents about their blood type. Write down each parent and yours if you know yours. Turn this in tomorrow for extra credit.

What do you know about blood types?

Multiple Alleles More than two allelic forms of a gene that

code for a trait Phenotype is still determined by a pair of

alleles! The gene pool simply contains more than two.

These alleles can be expressed in various ways: Complete dominance Co-dominance Incomplete dominance

Multiple Alleles: Human Blood TypeBlood Type is Coded by Multiple Alleles A, B, and O.

*A and B are co-dominant.*A and B are completely dominant over O.

PhenotypesType AType AType BType B

Type ABType O

GenotypesAAAOBBBOABOO

*The genes cause the production of a cell surface protein A or B.Type O cells have neither protein, so Type O individuals are Universal DONORS. Which individuals are Universal RECEPIENTS (have both proteins)?

Type A

Type OType AB

Type B

No antibodies

Multiple AllelesMany genes are present in 3 or more versions (alleles) – this is known as multiple alleles.

The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene.

The AB phenotype (genotype IA IB) is an

example of codominance!!

Codominance

The human ABO blood group illustrates another genetic

phenomenon – codominance.

Codominance occurs when the phenotype associated with each allele is expressed in the

heterozygote – both are expressed!

Problem 1

1. In humans, the blood types A, B, AB and O are determined by three alleles. A man who has AB blood marries a woman with O blood. What are the genotypes and phenotypes of the offspring?

Problem 2

2. A man who is heterozygous type A marries a woman who is heterozygous type B. What percentage of their offspring will be Type 0?

Problem 3

3. If a man with blood type B, one of whose parents had blood type O, marries a woman with blood type AB. What percentage of their children should have blood type B?

Problem 4

4. A man with type O blood and a woman with type AB blood get married. One of their children needs an operation. This child has type B blood. Can the child safely receive a blood transfusion from either parent? If not, why?

Multiple Alleles: Whose Baby ?A major mix-up occurred. Have the babies been returned

to the correct set of parents?

Baby A - Type O Blood Baby B - Type B BloodSmiths - Types O and AB Jones - Types A and B

1

Baby C - Type AB Blood Baby D - Type A Blood2

Squares -Types AB and AB Angles -Types AB and O

1/13/11: Warm UpBoth Mrs. Smith and Mrs. Jones had babies the

same day in the same hospital. Mrs. Smith took home a baby girl, whom she named Shirley. Mrs. Jones took home a baby girl, whom she named Jane. However, Mrs. Jones began to suspect that her child had been accidentally switched with another baby in the nursery.

Mr. Smith – type A Mr. Jones – type AMrs. Smith – type B Mrs. Jones – type AShirley – type O Jane – type B

**Did a mix-up occur? SHOW YOUR WORK!!!

Sex – Linked Genes

Review: What are the two sex chromosomes? What is a woman’s genotype? What is a man’s genotype? Which do you think there are more of – x-

linked genes or y-linked genes? Who do you think inherits sex-linked

disorders more – males or females?

X-linked Genetics

• Genes that are located on the X chromosome but not the Y

• MALES receive only ONE copy

• FEMALES receive TWO copies

Color Blind Test

Are you red-green colorblind? Lets find out…

Sex- Linked Inheritance

A male always receives a sex-linked condition from his mother, from whom he inherits an X chromosome.

A female must receive 2 alleles, one from each parent, for the trait to be expressed.

X-linked disorders are often recessive.

Sex-linked ( X-linked) disorders

Color –blindness Hemophilia – protein for blood clotting is

missing Duchenne Muscular dystrophy-

progressive wasting of muscles

(There are about 205 X-linked recessive disorders)

Color-blindness

Male

Female

Mother (carrier)

Father(normal vision)

Daughter(normal vision)

Son(normal vision)

Daughter(carrier)

Son(colorblind)

More males than females are affected by this type of disorder. WHY?

True or False? The X chromosome carries alleles that are not on

the Y chromosome. Therefore, a recessive allele on the X chromosome is expressed in males.

Certain traits that have nothing to do with the gender of the individual are controlled by genes on X chromosomes.

Males have 2 X chromosomes, and therefore, X-linked recessive alleles are expressed.

Solving X-linked Problems

How do you know if x-linked? Know color-blindness and hemophilia are!

Make Punnet Square using XX and XY Attach alleles to X’s ONLY To help determine percentages, circle the

male offspring

Let’s Practice1. Both the mother and the father of a male

hemophiliac appear to be normal. From whom did the son inherit the allele for hemophilia? What are the genotypes of the mother, the father, and the son?

2. A woman is color blind. What are the chances that her sons will be color blind? If she is married to a man with a normal vision, what are the chances that her daughters will be color blind? Will be carriers?

Cont…3. A husband and a wife give birth to a baby

girl. Mom’s vision was normal, so she assumed the baby’s would be normal too. However, she gave birth to a color-blind daughter. What can you deduce about the girl’s parentage?

Cont…4. A man with normal color vision marries a

woman who carries the recessive gene, although her vision is normal. What percentage of male children will be colorblind? What about females?

Cont…5. If a hybrid female for the colorblind trait

marries a colorblind male, what percentages of offspring will be colorblind?

Paternity Suit6. One of the sons from question #5 is

involved in a paternity suit. The child in question is a cb female. The judge rules that the child could not possibly belong to this man. Which sons, of problem #5, was involved in this case? Give his genotype and phenotype and tell how you know.

Sex-Linked Problems

Finish for homework!!!!

Article Anticipation Guide

Read the statements on your guide and mark your educated guesses in the boxes

Skin Color and Genetics Article

Polygenic Inheritance – when a Single Trait is influenced by many genes

Examples:

1. Height 2. Hair texture

3. Skin color 4. eye color

Types of Traits – Single Gene

• Some traits are controlled by single gene

• Expressed as “either/or”

Discontinuous Variation

• Polygenic Traits are controlled by the additive effects of 2 or more genes

• Shows a range of phenotypes

Continuous Variation

Types of Traits – Polygenic Traits

Types of Traits Example: A Biology Class

Continuous VariationDiscontinuous Variation

Single Gene Trait Polygenic Trait

Human Population - Skin Color

Controlled by many genes

A given individual can have as many as six alleles (possibly more!) for pigmentation

Types of Traits – Polygenic Traits

Polygenics

What are other traits that are coded for by MANY genes????

Multifactorial Traits Influenced by genetic and environmental

factors

*Even with a the same genetic make-up, environment can affect the phenotype, as seen in the Hydrangea above. The color variation is a reaction to acid in the soil. Similar environmental influence is observed in humans such as skin color and body build, giving even identical twins slight differences.

Review Problems

Gametes: For each of the diploid genotypes presented

below, determine all of the possible haploid gametes.

a. Rr b. RrYy c. RrYyBb

In turkeys a R produces a bronze color and is dominant over red color. Another dominant gene H, results in normal feathers; its recessive allele h produces feathers without webbing. Two heterozygous bronze turkeys with normal feathers were mated.

What kind of problem is this? ___________________ Give the genotypes of P:_________________ Give the genotypes of F1:________________ Give the phenotypes of F1:_______________

Example 1

Example 2

In pea plants purple flowers are dominant to white flowers. If two white flowered plants are cross, what

percentage of their offspring will be white flowered? ______________

What kind of inheritance is this? __________ Monohybrid or dihybrid? ________________

Example 3

A cross between a blue-flowered plant and a white-flowered plant gives all pale-blue-flowered plants. What kind of inheritance is this?___________ What are the genotypes of the offspring?

_______ Monohybrid or dihybrid?_________________

Example 4 A father and mother claim they have been

given the wrong baby at the hospital! Both parents are blood type A. The baby they have been given is blood type O. Could they have the right baby? What kind of inheritance is this?___________ P genotypes: ______________

Example 5 In a certain cactus, prickly spines can be

two-pronged or one-pronged. If a one-pronged cactus is crossed with a two-pronged cactus, the F1 generation has a both types of spines, some are two-pronged, some are one-pronged. What kind of inheritance is this?___________ Monohybrid or dihybrid?_________________ Give the genotypes of P:_________________ Give the genotypes of F1:________________

Example 6

Located on the X chromosome of a cat is a gene that codes for deafness. This gene is recessive. A black female cat that is is heterozygous for deafness (Dd) is crossed with a yellow male cat that is not deaf. What type of problem is this?_____________ P genotypes:__________________________

Human Traits

Genetic counselors analyze how the trait is passed from one generation to the next. They construct pedigree charts that show the pattern of inheritance for a particular condition, and show if a disorder is dominant or recessive.

Pedigree Chart

Genetic Disorders Autosomal ( non-sex linked) Dominant-

individuals with alleles AA or Aa will have the disorder.

Autosomal Recessive – individuals with alleles aa with have the disorder.

Autosomal Disorders

Do you remember what an autosome was?

Autosomal Dominant Disorders:- Dwarfism known as achondroplasia- Huntington’s disease

Autosomal Recessive Disorders:- Phenylketonuria ( PKU)- Tay-Sachs disease- Cystic Fibrosis- Albinism Incompletely dominant disorders:- Sickle-Cell disease

Ways to recognize autosomal dominant disorders:

Affected children usually have an affected parent.

Two affected parents can produce an unaffected child.

Two unaffected parents will not have affected children.

Inheritance Pattern I:

Ways to recognize autosomal recessive disorders:

Most affected children have normal parents. Two affected parents will always have

affected children. Affected individuals with homozygous normal

mates will have normal children.

Inheritance Pattern II:

Dominant or recessive?




Sex – Linked Pedigrees

Y – linked (ONLY males will inherit) Male infertility

X – linked Hemophilia Red/Green Color Blindness MS

Pedigree Analysis

http://www.knowledgene.com/public/view.php3?db=gene_school&uid=35

http://www.knowledgene.com/public/view.php3?db=gene_school&uid=35

Dihybrid Crosses - looking at 2 traits

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