Mendel Extensions

Incomplete Dominance

Results in an intermediate phenotype (e.g., pink flowers

from red and white parents).

Codominance

Both alleles are expressed equally (e.g., spotted patterns in

animals, ABO blood groups)

Polygenic Traits

Traits influenced by multiple genes, leading to a continuous

range of phenotypes (e.g., human height).

Epistasis

Interaction between genes where one gene's activity depends on

another's (e.g., coat color in dogs).

Pleiotropy

A single gene influencing multiple traits (e.g., clasp-1 mutants in

plants causing varied developmental issues).

1. Incomplete dominance

This is basically a “dosage” phenomenon. One functional allele means half the

amount of protein, which means half the work gets done.

2. Codominance

Codominance means what it sounds like: both alleles are dominant, so both

phenotypes are present in heterozygotes.

o Examples: magnolia flower, Speckled chickens, spotted cows. In all of these

cases, there are patches of two different colors in heterozygotes, each color

corresponding to one of the alleles.

o Other example is A/B/O blood types

3. Polygenic traits

Most traits are the result of the combined activity of many genes, so they do not

transmit across generations in a Mendelian manner (such as disappearing in F1

and reappearing in F2).

o Most traits are polygenic

4. Epistasis

Epistasis is when a gene’s activity is dependent on another gene’s activity.

o Think of a biological sequence of events (like a metabolic pathway or signaling

pathway)

o Analogy is an assembly line, where if the person one step before you calls in sick,

then you have nothing to work with. In that case, if you also call in sick (or

anybody else after you), (i.e. you are mutated and lose function), there is no

difference in the outcome. The final product still does not get made.

5. pleiotropic genes

In this case, one gene is responsible for many different traits.

o This is because it is involved in many pathways, each of which may be influenced

in different ways by the gene.

o So it is not obvious that all the traits are related.

If two genes are on the same chromosome, they are ……..

linked

Locus

the location of a gene on a chromosome

If the genes are linked, their traits are ……..

linked

Linked genes don’t assort __________.

independently

The closer together the two gene loci are……

the tighter they are linked!

Why does this happen?

Because linked genes can only be separated by random crossovers during meiosis

genes far apart

cross-overs more likely,

more recombinants

genes close together

cross-overs less likely,

less recombinants

Linked genes can only be separated by crossovers during…..

meiosis

genes far apart (high or low probability of cross overs?)

high probability of cross-overs

genes close together

low probability of cross-overs

If two genes are on the same chromosome, they do not sort independently. They

are……

linked

Because of this, the ratios that we observe with two unlinked genes (9:3:3:1) DO OR DO

NOT occur.

DO NOT

The two traits will be present on a given individual unless a ________________ between the two genes.

crossover during meiosis

The farther apart the two genes are from each other on the chromosome, the

more or less likely a crossover is to occur.

more

The closer they are together, it becomes less and less frequent the traits are

separated, meaning they are ….

linked more tightly.

Geneticists need to know….

how close two loci are to one another, i.e. how tightly

they are linked.

How do they do it?

Easiest way is to use testcross between double homozygotes and look at the

degree of deviation the expected progeny ratios (25% each combo).

Dihybrid testcross of two unlinked genes

testcross progeny segregate 1:1:1:1

(i.e. 25% each genotype)

Testcross of two linked genes

testcross progeny ratio

deviates from 1:1:1:1

o (i.e. deviates from 25% each)

o determine the distance between two genes on

the chromosome, we calculate the……

recombination

frequency, the percentage of testcross progeny

that are recombinants.

Since the distances are relative, they don’t tell us

anything about actual physical distance between

the genes (number of base pairs).

So they actually quantify?

frequency of recombination

1% recombination = 1 map unit = 1 _________

centimorgan

Genetic linkage patterns can be used to determine….

-the order and distance between linked genes.

-Basically, the closer the two genes are together on the chromosome, the more

likely those two traits will be present on an individual.

recombination can’t be more than 50% because…

o during meiosis, the

gametes are ½ parental

and ½ recombinant.

o So those will always

make up at least half of

the testcross offspring.

The more genes you know, the merrier, because

As two genes get farther apart, they recombination frequency gets skewed a bit

lower.

Why does the recombination frequency get lower?

Due to double crossovers between two genes.

Explain the process of the crossing over.

-Crossing twice would flip the allele back onto the parental chromatid.

-The alleles would effectively be indistinguishable from independent assortment.

Genes that are found on the X chromosome only are called…..

X-Linked

Are males or females more likely to show the recessive trait?

Males are more likely to show the recessive trait of a gene because they only

have one copy of the gene, whereas females have two copies, so need to be

homozygous recessive to exhibit the recessive trait.

Females need to be _____________ to exhibit the recessive trait.

homozygous recessive

A. Human X chromosome (2 points)

1)Large (2350 genes)

2)Most X-linked genes are nonsexual traits.

B. Human Y chromosome

1)Small (few genes)

2)Very few match genes on X chromosome

3)Contains SRY gene

▪ Regulates expression of genes that trigger male development

With X-linked genes, your genotype will depend on….(2 points)

whether you are male or

female, and whether you get the gene from your mother or father

X-linked recessive traits more common in…

males!

such as…

1)Red−green colour blindness

2)Hemophilia: Defective blood-clotting protein

Dosage compensation in female mammals of inactivation of one X chromosome

In female mammals, inactivation of one X chromosome makes the dosage of X-

linked genes the same as male

When does this occur?

Occurs during embryonic development

o Same X chromosome inactivated in ………………….

all descendents of a cell

/What does the inactivitaion result to?

Results in patches of cells with different active

X chromosomes

What is the Barr Body (2 points)

-Inactive, tightly coiled condensed X chromosome

-Copied during mitosis but always remains inactive