Monohybrid crosses and Sex linkage

Monohybrid cross definition

One gene at a specific locus controlling a single trait

Remember, a diploid organism has __ of chromosomes, called ___ chromosomes. One chromosome is the __, and the other is the __.

Pairs, homologous.

Maternal, paternal.

Allele def

Variation of a gene at a specific locus between chromosomes in a pair.

Phenotype def

Complex interactions between many genes and the environment

Incomplete dominance def

Heterozygotes have an intermediate phenotype

Codominance def

Heterozygotes express BOTH traits (blood types)

Penetrance def

Percent of individuals with a given phenotype that have the EXPECTED phenotype

AKA, zero expressivity

Expressivity def

The varying severity of the phenotype

Mendel’s homozygous pea plants were called..

pure breeding

What was Mendel’s first pea cross and what were its outcomes?

• Homo aa: green

• Homo AA: yellow

F1 generation: all Aa, yellow

What were the outcomes of Mendel’s F1 heterozygous cross?

3:1 Yellow to green

1:2:1 for AA, Aa, and aa

What principles did Mendel give us?

Segregation of chromosomes, dominance, independent assortment

Segregation def

Separating of the homologues and alleles into different gametes

Outcome of an AAxAa cross

• 50% AA

• 50% Aa

Outcome of an Aa x Aa cross

• 50% Aa

• 25% AA

• 25% aa

3:1 phenotypes if dom/rec

Outcome of an Aa x aa cross

• 50% Aa

• 50% aa

50/50 phenotype split if dom/rec

What is independent assortment?

Homologous chromosomes can orient either direction on the metaphase plate, disregarding how the other pairs orient

How many different gametes (genotypes) will be produced by am organism with genotype AaBbCcDdEeff?

2⁵ → NOT 6, despite 6 positions, because ff is homozygous

Why does penetrance and expressivity happen

• Other genes and variations

• Biological environment

• Environmental factors

What are the 4 different kids of sex determination we learned about?

1. Mammals: F is XX, M is XY → presence of Y chromosome

2. Flies: Dosage of X chromosomes, anything over 1 is F, under is M

3. C. elegans: No Y chromosome, XO are M, XX is F and is self fertilizing

4. Birds: Opposite of mammals in a way, F is heterogametic (ZW), Male homogametic (ZZ)

What does heterogametic mean? What about homogametic?

• Hetero: two different sex chromosomes

• Homo: same sex chromosome

What are some other forms of sex determination?

• Temperature dependent (reptiles)

• Behavioral cues (clown fish)

  • If dominant female dies, the dominant male will change its sex and become the new dominant female

What are some common examples of X-linked traits?

• Eye color in flies

• Coat color in cats

• Color-blindness

• Hemophilia

• Male pattern baldness

How did Thomas and Lily Morgan discover the first sex-linked trait with fruit flies?

1. In a population of fully red-eyed flies, there was one white eyed male

2. When the male was crossed with a red-eyed female, all of the resulting F1 flies had red eyes → red dominates white

3. When the F1s were crossed, all of the female progeny had red eyes. BUT, half of the males had white eyes!

T or F: Dad’s will ALWAYS pass their x chromosome onto the daughter

TRUE!

Males are more susceptible to ____ because they only have one X chromosome

X-linked recessive

T or F": a male’s X chromosome came from his mother

T!

What is dosage compensation?

One sex has more X’s than the other, so there must be a solution to level the expression of X chromosomes between sexes (evolution)

What is mammal’s form of dosage compensation?

Inactivate one x in the homogametic sex

What are flies’s form of dosage compensation?

Up-regulate the X in the heterogametic sex

What is C. elegans form of dosage compensation?

Down regulate both Xs by 50% in the homogametic sex

What is responsible for the female calico cat pattern?

Random inactivation of an X chromosome in every cell during early development makes daughter cells with that same inactivation, which forms patches of like cells, ultimately leading to the coat pattern we’re familiar with

What is the molecular mechanism behind mammal’s dosage compensation method?

1. XIST RNA is expressed at a higher level from one X chromosome (randomly)

2. XIST RNA coats chromosome and marks it for inactivation

3. Additional modifications occur to inactivate the X

T or F: nondisjunction of the sex chromosomes is just as harmful as non disjunction of the autosomes

F!!

But you do need AT LEAST ONE X to survive!

List the 3 different conditions associated with non-disjunction of the sex chromosomes

1. Turner’s XO

2. Klinefelter’s: XXY

3. Jacob’s: XYY

T or F: if a mother has a homozygous recessive X-linked trait, all of her sons will have that trait

TRUE! Son’s inherit at least one copy of an x chromosome from their mom