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 (2 n ) appear in the F
2 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.
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

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

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

How many gametes?
To determine the number of different gametes a parent can have use the FOIL method.
Number and kind of gametes
TTYY
= one TY
TtYY
= two TY and ty
TtYy
= four TY, Ty, tY, ty

Possible Gametes (sperm/egg):
If I cross:
What are the different possible gametes for these parents??
RRTT x RrTt
RT RT, Rt, rT, rt
There are 4 total different gametes

If I cross:
What are the possible gametes?
RRTT x RrTt
RT
RT
Rt
RRTT RRTt rT
RrTT rt
RrTt
I have 4 possible offspring

If I cross:
What are the possible gametes and offspring?
rrTt x RRTt
RT Rt rT
RrTT RrTt rt
RrTt Rrtt
I have 4 possible offspring

If I cross:
What are the possible gametes?
RrTt x RrTt
RT Rt
RT
RRTT RRTt
Rt
I have 16 possible offspring
9:3:3:1 rT rt
RRTt
RrTT
RrTt
RRtt
RrTt
Rrtt rT
RrTT
RrTt rrTT rrTt rt
RrTt
Rrtt rrTt rrtt

Why Did Mendel Conclude That The
Inheritance of one Trait is Independent of Another?
Phenotype Ratio:
9 yellow/smooth
3 yellow/wrinkled
3 green/smooth
1 green/wrinkled
Because it’s the only way to explain the pattern of inheritance!

What Works for Peas Also
Works for
Humans
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.

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

 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, ty b. TY, ty c. TY, Ty, ty d. Ty, ty, tY, TY

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?

 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

Blood Type is Coded by Multiple Alleles A, B, and O .
*A and B are co-dominant.
*A and B are completely dominant over O.
Genotypes
AA
AO
BB
BO
AB
OO
Phenotypes
Type A
Type A
Type B
Type B
Type AB
Type O
*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 AB
No antibodies
Type B
Type O

Multiple Alleles
Many 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
( I A , I B , and i ) of a single gene.
The AB phenotype
(genotype I A I B ) 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!

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?

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?

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?

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?

A major mix-up occurred. Have the babies been returned to the correct set of parents?
1 Baby A - Type O Blood Baby B - Type B Blood
Smiths - Types O and AB Jones - Types A and B
2
Baby C - Type AB Blood Baby D - Type A Blood
Squares -Types AB and AB Angles -Types AB and O

Both 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
Mrs. Smith – type B
Shirley – type O
Mr. Jones – type A
Mrs. Jones – type A
Jane – type B
**Did a mix-up occur? SHOW YOUR WORK!!!

 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 – xlinked genes or y-linked genes?

Who do you think inherits sex-linked disorders more – males or females?

•Genes that are located on the
X chromosome but not the Y
•MALES receive only ONE copy
•FEMALES receive TWO copies

 Are you red-green colorblind?
 Lets find out…

 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 dystrophyprogressive wasting of muscles
(
There are about 205 X-linked recessive disorders)

Father
(normal vision)
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Mother
(carrier)
Daughter
(carrier)
Son
(colorblind)
More males than females are affected by this type of disorder.
WHY?

 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, Xlinked recessive alleles are expressed.

 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

1. 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?

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?

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?

5. If a hybrid female for the colorblind trait marries a colorblind male, what percentages of offspring will be colorblind?

6. 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.

 Finish for homework!!!!

 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

• 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

Single Gene Trait
Polygenic Trait
Discontinuous Variation Continuous Variation

Types of Traits – Polygenic Traits
 Controlled by many genes
 A given individual can have as many as six alleles ( possibly more!
) for pigmentation

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

 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.

 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:_______________

 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? ________________

 A cross between a blue-flowered plant and a white-flowered plant gives all pale-blueflowered plants.

What kind of inheritance is this?___________

What are the genotypes of the offspring?_______

Monohybrid or dihybrid?_________________

 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: ______________

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

 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:__________________________

 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 .

 Autosomal ( non-sex linked) Dominantindividuals with alleles AA or Aa will have the disorder.
 Autosomal Recessive – individuals with alleles aa with have the disorder.

 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

 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:

 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?

Dominant or recessive?

Dominant or recessive?

Dominant or recessive?

 Y – linked (ONLY males will inherit)

Male infertility
 X – linked

Hemophilia

Red/Green Color Blindness

MS

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