Biology Genetics Part 2

Biology Genetics Part 2 • Objectives: a) Review Mendelian Genetics • b) Types of cells/Types of Chromosomes • c) Sex linked traits • d) Sex Influenced traits • e) Polygenics • f) Epistasis

Cells • We have 2 types of cells • 1. Somatic Cells (2n) – body cells • 2. Sex Cells (n) – sperm and egg

Chromosomes • We have 2 types of Chromosomes: • 1. Autosomal chromosomes – pairs 1 through 22 in humans (non-sex chromososmes) • 2. Sex chromosomes – xx or xy • What about a fruit fly with a 2n = 8?

Sex-linked Traits

Definition • Sex-linked traits are traits that are controlled by genes on the sex chromosomes • The X and Y chromosomes

Are they controlled by both sex chromosomes? • Most sex-linked traits are controlled by genes on the X chromosome. • This is because an X chromosome is much larger than a Y chromosome. • A few traits are suspected to be controlled by genes on the Y chromosome, however there is less research about Y-linked traits.

Different Forms of Sex-linked Inheritance • There are three different forms of sex-linked inheritance that we will be examining: • X-linked recessiveinheritance • X-linked dominant inheritance • Y-linked inheritance

X-linked Recessive Inheritance • X-linked recessive traits are traits resulting from a recessive allele on the X chromosome. • There are over 100 different human conditions that are caused by recessive alleles found on the X chromosomes. • X-linked recessive alleles are represented by a X , superscript lower case letter or a plus/minus • (+/-)

X-linked Recessive Inheritance • These traits tend to show up in males more than females. • Why? Because males only carry one X chromosome! ( they have one x chromosome, and one Y chromosome that make up their sex chromosomes).

X-linked Recessive Inheritance • Example #1: Colorblindness • The allele that controls colorblindness is found on the X chromosome and is recessive . • What genotype must a female have to be diagnosed as colorblind? Homozygous recessive (X-X-)

Are you colorblind? Normal Color Vision:A: 29, B: 45, C: --, D: 26 Red-Green Color-Blind:A: 70, B: --, C: 5, D: --3. Red Color-blind:A: 70, B: --, C: 5, D: 64. Green Color-Blind:A: 70, B: --, C: 5, D: 2

Try this problem! • What is the probability that the sons of a homozygous recessive mother would be colorblind? • Represent colorblindness with a “-” Mothers Genotype X- X - Each son would receive an X chromosome from their mother which means… Each son would have an X chromosome with a recessive allele for colorblindness and would express the trait!

X-linked Recessive Inheritance • Example #2: Hemophilia Hemophilia • Hemophilia is a serious ailment in which the blood lacks a clotting factor, and therefore when an individual is injured, they cannot stop bleeding. • Hemophilia is also controlled by recessive alleles on the X chromosome.

Try this problem • A woman who is heterozygous (a carrier) for hemophilia marries a normal man: • a. What are the genotypes of the parents? • b. Make a Punnett square for the above cross. • c. What is the probability that a male offspring will have hemophilia? __________ • d. What is the probability of having a hemophiliac female offspring? _________

X-linked Dominant Inheritance • X-linked dominant traits are traits that result from the presence of a dominant allele on the X chromosome. • Unlike X-linked recessive traits, females and males both require only ONE dominant allele in order to express the trait. • X-linked dominant traits are represented by an X, superscript capital letter or a (+)

X-linked Dominant Inheritance • Example: Faulty Tooth Enamel and Dental Discoloration • Individuals who have an X chromosome that carries a dominant allele for this trait will have dental discoloration.

Try these problems! • Would a heterozygous woman for dental discoloration display the trait? YES! Because this is a X-linked dominant trait, you only need to carry one dominant allele in order to express the trait.

Try these problems! • What percentage of the children from a heterozygous mother and an affected father would have dental discoloration? • Represent dental discoloration with a “D” All of the daughters will display the trait as they each carry at least one dominant allele. One son will demonstrate the trait while the other won’t.

Y-linked Inheritance • Y-linked traits are controlled by alleles on the Y chromosome • Another word for Y-linked traits is holandric traits as they are “wholly male”.

Y-linked Inheritance • Do the words homozygous or heterozygous apply to Y-linked traits? • Are females affected by Y-linked traits? No, because there is only ever one Y chromosome present at a time! No, because females don’t carry Y chromosomes!

What is this?

Y-linked Inheritance • An example of a Y-linked trait is . . . . . HAIRY EARS!

Y-linked Inheritance • All of the sons of an affected male will display this Y-linked trait.

A final example of Sex-linked Traits • Eye color in fruit flies ( Drosophilia melanogaster ) • Eye color is controlled by the X chromosome. • Red eyes are dominant to white. • White eyes are most common in males. • Females only display white eyes if they are homozygous recessive for the trait.

Fruit Flies Continued • What type of X-linked inheritance is this? • Try your new knowledge out on the handout you are about to receive! X-linked recessive Inheritance because the males show the trait more often that the females. Also, the females must have two X chromosomes, both of which carry the recessive alleles for white eye color in order to have white eyes!

Let’s Try These • Problems • Practice Problems

Biology Genetics Part 2