Topic 3: Sex-Linked Inheritance and Gender Determination

Topic 3: Sex-Linked Inheritance and Gender Determination
Due Thursday, May 20th

This session we will explore inheritance patterns associated with genes located on the sex chromosomes. We will also look at the role that sex chromosomes play in sex determination and gender.

Assignment 1: Battle of the Sexes

Genes physically located on the sex chromosomes are called sex-linked genes. Genes found on the X chromosome are X-linked, while genes on the Y chromosome are Y-linked. Only males inherit Y-linked genes, since females do not have a Y chromosome. Males inherit their Y chromosomes from their fathers.

Males inherit one X chromosome from their mother, and are called "hemizygous" for a given X-linked trait. Females inherit two X chromosomes (one from mom, one from dad) and can be either homozygous or heterozygous for a given X-linked trait.
Examples of X-linked recessive disorders include muscular dystrophy, hemophilia, and red-green colorblindness.

A male who inherits a recessive disorder allele on his X chromosome will exhibit that disorder, since he does not have a second X chromosome (containing a normal allele) to mask the disorder allele. As a result, there are no male carriers for X-linked disorders. On the other hand, females can be carriers for X-linked recessive disorders since they inherit two X chromosomes (and the second X chromosome might contain the normal allele).

For this assignment, we will explore X-linked recessive inheritance patterns.

To begin:

Read:

Text pages 358-364.

Part A: Sample Problems to Try:

1) Colorblindness is an X-linked recessive trait. If a colorblind male and a homozygous normal female have children, will any of the children be colorblind? Show the Punnett square.

2) Colorblindness is an X-linked recessive trait. If a normal male and a colorblind female have children, will any of the children be colorblind? Show the Punnett square.

Sites to Visit:


Part B: Sex-Linked Inheritance Tutorials and Practice Problems
http://www.biology.arizona.edu/mendelian_genetics/problem_sets/sex_linked_inheritance/sex_linked_inheritance.html

WRITE DOWN THE ANSWERS TO QUESTIONS 1-10 AND TURN THEM INTO MS. JOHNSON ON THURSDAY, MAY 20TH!!!


Part C: Complete the worksheet you were given in class, "Sex Linked Inheritance"


So...due Thursday, May 20th

-Part A: 2 simple sample problems
-Part B: Answers to online tutorial practice questions 1-10
-Part C: Sex-linked Inheritance worksheet you received in class.

Topic 2: Extensions and Exceptions to Mendel's Laws of inheritance-PART ONE

DUE FRIDAY, MAY 7TH

Assignment ONE---Complete part 1 and 2

This session will will explore many of the "extensions and exceptions" to Mendel's Laws of inheritance.

Assignment 1: Who's baby is it?

Human blood type genetics provides another example of multiple gene inheritance. There are several genes that influence blood types, in particular, the ABO and Rh factor genes.

For the ABO gene, we can have A blood, B blood, AB blood or O blood.

For the Rh gene, we can have Rh+ or Rh-.

Taking these together, we can have A+, A-, B+, B-, AB+, AB-, O+, or O- blood.

The ABO gene also provides an example of codominance. There are three alleles for this gene: the A allele, the B allele, and the O allele. Both A and B alleles are dominant (so there are 2 forms of the dominant gene, the A form and the B form), while O is recessive. Thus, if a person inherits an A allele and a B allele, they would have AB blood.

The only way to have O blood is to inherit two O alleles (ii).

To begin this assignment, complete the following:

Sites to Visit:
How Stuff Works: Blood Types
http://health.howstuffworks.com/question593.htm

Blood Types Tutorial and Practice Problems
http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/Intro.html

Blood Typing: See Introduction and Blood Typing game
http://www.nobel.se/medicine/educational/landsteiner/index.html

Part 1: Blood typing game answers
WHILE COMPLETING THE BLOOD TYPING GAME, ANSWER THE FOLLOWING QUESTIONS!!!
1) What is the bloodtype of the man with the purple mohawk?
2) What is the bloodtype of the red haired woman???
3) What is the bloodtype of the old balding man???


Part 2: Sample Blood-Typing Problems: (Complete the following)

A) Show the Punnett square of a cross between a man who is homozygous for A blood with a woman who is homozygous for B blood. Give the phenotype(s) and the ratios of the possible children.

B) Show the Punnett square of a cross between a man who is heterozygous for A blood with a woman who is heterozygous for B blood. Give the phenotype(s) and the ratios of the possible children.

Topic 1 Notes

Topic 1 Notes

Gregor Mendel
Father of modern genetics; combined plant breeding, statistics, careful record-keeping; described hypothesis of transmission of traits now considered the laws of inheritance. Mendel studied pea traits with two distinct forms:

For example, Mendel studied pea plant height:
• Mendel observed that crossing tall x tall yields tall plants.
• In other cases, he observed that tall x tall yields both tall and short plants.
• Mendel concluded that some plants were "pure" and others were "hybrids."
• Hybrid plants carry the recessive short trait (which is hidden, but not absent) and show the dominant tall trait.
• The observed trait is dominant; the masked trait is recessive.

Phenotype Genotype Description
Tall plant TT Homozygous dominant ("tall"-associated alleles)
Tall plant Tt Heterozygous (sometimes called a carrier)
Short plant tt Homozygous recessive ("short"-associated alleles)

Alleles = Versions of the same gene or DNA sequence; the DNA sequence differs at one or more sites.
• Homozygous: Cell contains two alleles that are the same (TT or tt).
• Heterozygous: Cell contains two alleles differ (Tt).
• Dominant: An allele that produces an effect even when present in just one copy; represented by a capital letter.
• Recessive: An allele that needs to be present on both chromosomes (two copies) in order to be expressed; represented by a lower case letter.

Phenotype = The observed trait or physical appearance.

Genotype = The combination of alleles present; the underlying instructions.





Law of segregation = States that parental alleles separate into different gametes during meiosis, such that each gamete contains only one allele for each trait. Parental cells have two alleles; gametes have one allele.

Punnett square
Used to predict the possible offspring that result from two parents. These predictions can be represented as ratios. It is important to note that the observed (real-life) ratios may not match the calculated ratios when looking at small numbers of offspring.

Monohybrid cross (Tt x Tt)

Two heterozygous tall parents (Tt) will produce gametes that contain either:
T (dominant, tall allele)
t (recessive, short allele)

A Punnett Square is used to predict potential offspring arising from this cross (Tt x Tt):
Offspring ratios
Genotypes: 1/4 TT : 2/4 Tt : 1/4 tt
Phenotypes: 3/4 tall : 1/4 short
The recessive short allele (t) is "hidden" in the heterozygote (Tt) offspring.

Modes of inheritance = Describes the patterns through which a particular phenotype (such as a disorder) is inherited.
• Autosomal dominant inheritance
• Autosomal recessive inheritance
• X-linked dominant inheritance
• X-linked recessive inheritance
• Y-linked inheritance


Autosomes are the "non-sex" chromosomes found in both genders.

Sex chromosomes (X and Y) are gender-specific. "X-linked" inheritance or "Y-linked" inheritance simply means that a gene is located on the X or Y chromosome, respectively. Sex-linked inheritance will be discussed more later on...

Autosomal dominant inheritance
For the inheritance of a disorder that is DOMINANT:
• Heterozygotes (Aa) inherit the dominant allele and exhibit the "affected" phenotype. Since the disorder is dominant, only one copy is needed; the recessive "unaffected" allele (a) is masked by the dominant "affected" allele (A).
• Males and females are equally affected since it is not associated with a sex chromosome' males and females may equally transmit the trait.
• The affected phenotype does not skip a generation. If the dominant allele is passed on, that offspring will express the affected phenotype.

Example: Parents: Aa x aa
Only Aa is affected by the dominant disorder (A), since aa has two recessive/normal alleles.
Genotypic ratios of potential children:
2/4 Aa : 2/4 aa
Phenotypic ratios of potential children:
2/4 affected (Aa) : 2/4 unaffected (aa)
50% affected : 50% unaffected

Autosomal recessive inheritance
For the inheritance of a disorder that is RECESSIVE:
• Heterozygotes (Cc) carry the recessive (affected) allele but exhibit the dominant (unaffected) phenotype. Since the disorder is recessive, two recessive alleles (cc) are needed in order to observe the disorder.
• Males and females are equally affected and may transmit the trait.
• The expression of the recessive "affected" trait may appear to skip generations if it is masked by a dominant "unaffected" allele.

Example: Parents: Cc x Cc (carriers)
Both parents are unaffected since they each have one dominant normal (C) allele. They are carriers of a recessive disorder.
Genotypic ratios of offspring:
1/4 CC : 2/4 Cc : 1/4 cc
Phenotypic ratios of offspring:
1/4 unaffected : 2/4 carriers : 1/4 affected
75% unaffected : 25% affected