Genetics Lecture 4

Fractions and perecent versus ratio

-Fractions (or sometimes %) are used to describe the frequency of a genotype or phenotype in a population or test cross.

-Ratio describes the relationship between differing genotypes or phenotypes in a population or test cross.

There are two fundamental rules that explain how genes are passed from parents to offspring.

-Genes are present on homologous chromosomes

-Chromosomes segregate and assort independently

How does gene interaction affect phenotypes?

Single phenotype is affected by more than one set of genes

X-Linkage

genes that are present on the X chromosome

Mutation

-Ultimate source of alleles. Without mutations, everyone would have identical genes. No variation.

-New phenotypes arise when mutation affects gene products of DNA.

Wild-type (wt) allele

Occurs most frequently in nature and is usually, but not always, dominant.

"normal" trait

Loss-of-function mutations

-a genetic change that reduces or completely abolishes a gene's normal activity, leading to a non-functional or less effective gene product

-A mutation causes loss of wild-type function

Gain-of-function mutations

Mutation enhances function of wild type • Quantity of gene product increases

Neutral mutations

No change to the phenotype • No change to the evolutionary fitness of the organism

Denoting alleles with subscripts

Avoids the assumption that there is dominance.

Complete dominance

-Phenotype of the heterozygote is the same as the phenotype of the homozygote.

Incomplete dominance

-Phenotype of the heterozygote is intermediate between the phenotypes of the homozygotes

-Appears as a blend of the phenotypes

-Creates a third phenotype

Codominance

-Phenotype of the heterozygote includes the phenotypes of both homozygotes

-Both alleles are visible in the phenotype – but NOT blended like in incomplete dominance

-Creates a third phenotype

Multiple alleles

A gene can have many different alleles in a population. Even if that is the case, a person can still only inherit 2 alleles for that gene.

Since there are many genes in that population, there are more possible allele pairings. (genotype) and more possible trait outcomes. (phenotype.)

Possible alleles in a blood group

I ^A

I^B

i

Dominance

I ^A >i

I ^B > i

I^A = I^B

Lethal alleles

-Yellow coat color is caused by a recessive lethal gene.

-Causes death early in development so the genotype never appears.

Recessive lethal

-Needed to be homozygous to be lethal. A heterozygote will not be lethal.

Ex) yellow coat color of mice

Dominant lethal

-Lethal in both homozygotes and heterozygotes

-Not common in population because individuals usually die before they can pass on genes

Ex) huntingtons disease

Conditional lethal

-Only lethal under certain conditions

-Unlike “classic” lethal alleles that always cause death (like YY in yellow mice), conditional lethals depend on the environment or trigger.

Ex)Favism- X linked lethal conditions, lethal if you eat fava beans

Yellow Coat Color

- Y is the dominant allele for yellow color.

- Y is also a lethal recessive allele.

-Y is behaving as a recessive lethal allele but is dominant with respect to the coat color phenotype.

What happens when a Yy and Yy are crossed under these conditions:

Y is the dominant allele for a yellow color.

2/3 are Yy so they get the yellow trait.

¼ are YY so they are yellow but also are dead because of the recessive lethal allele.

¼ are non-yellow because they do not inherit the yellow coat color allele.

What happens when you combine two different modes of inheritance

-It modifies the 9:3:3:1.

-Consider the inheritance of albinism (simple recessive) AND blood type (multiple alleles) • This dihybrid cross does not yield four phenotypes in the classical 9:3:3:1 ratio. Instead, six phenotypes occur in a 3:6:3:1:2:1 ratio

Gene interaction

-More than one gene can affect a single trait.

-The effects of one gene at a locus depends on what other genes are present.

-Genes exhibit independent assortment of alleles. BUT do not act independently in their phenotypic expression.

-When different genes work together, the trait you see (phenotype) can be different from what each gene would produce on its own.

• New phenotypes → the visible trait is a result of the combination, not just one gene.

• Not predictable from single-locus effects → you can’t figure out the final trait just by looking at each gene individually

Epistasis

One gene hides the effect of another gene at a different locus

Epistatic gene

gene that masks the other

Hypostatic gene

gene whose effect is masked

Recessive epistasis

the epistatic gene masks the hypostatic gene when in recessive form

Dominant epistasis

the epistatic gene masks the hypostatic gene when in dominant form

Complementation analysis

When two mutations produce a similar phenotype, they may either be in the same gene or in different genes.

Complementation analysis test (definition only)

helps determine if two recessive mutations that produce the same phenotype are in the same gene (alleles) or in different genes.

Why is it useful?

By testing many mutations for a trait, you can figure out how many genes are involved in a trait.

Principle

If two recessive, mutant parents produce offspring with a normal (wild-type) phenotype, the mutations are in different genes because each parent provides a functional, normal copy of the other's defective gene. This means they complement. (Case 1)

-(different gene/locus)

Failure to Complement:

If the offspring (F1) exhibit the mutant phenotype, the mutations are in the same gene, meaning neither parent provided a functional, normal copy of that gene. (Case 2)

(same gene/locus)

Requirements to the complementation analysis test

The mutations being tested must be recessive.

Pleiotropy

-When a single gene influences multiple phenotypic traits.

Ex)Chickens with the frizzle trait have lower egg yields, higher metabolic rates and digestive capacity, and abnormal body temperature

Sex Linked characteristic

-Characteristics determined by genes located on sex chromosomes

X-linked

genes on x chromosome determines characteristic

Y linked

genes on Y chromosome determines characteristic

Who will inherit from an infected dad with a mutation on his y sex chromosome

Only sons because daughters do not have a y chromosome.

Most sex linked traits will be “___”? Why?

X linked. because a y chromosome has less information than an x chromosome.

X linked recessive trait

• Males (XY) have only one X → if they inherit the recessive allele, they show the trait.

• Females (XX) have two X’s → need two copies of the recessive allele to show the trait.

• Affected males cannot pass the trait to sons (they give Y to sons).

• Affected males pass the allele to all daughters, making them carriers if the mother is normal.

X linked dominant traits

Only one copy of the allele is needed for the trait to be expressed.

Sex limited characteristics

Autosomal gene expressed in ONLY one sex

Sex influenced

Autosomal genes expressed differentially in males and females. The expression of the genes is dependent on hormone constitution of the individual. The opposing alleles exhibit the same phenotype in opposing sexes.

What may alter phenotype expression?

Genetic background and the environment

Gene expression

Expression depends on both genotype (includes other interacting genes) and environment.

Penetrance

the likelihood that someone with a certain genotype will actually show the trait.

If 100 people have the same genetic mutation for a trait, but only 80 of them actually show the trait, the penetrance is 80%.

Incomplete penetrance:

when a genotype does not produce the expected phenotype

Caused by other genes or environmental factors

Example of penetrance

How many people have the allele for polydactyly? 42 • How many people have polydactyly? 38 • 38/42= penetrance of polydactyly → incomplete penetrance

Expressivity

Even though two people might express the same gene, there are levels at which that gene can be expressed.

This includes functionality, size, and number.

The position affect

The physical location of a gene can influence how it is affected.

For example, if a gene is moved near heterochromatin, which is condensed and genetically inert, its expression can be silenced.

Conditional mutations

An example is temperature. Temperature sensitive mutations are an example of a conditional mutation • Siamese cats and Himalayan rabbits, exhibit dark fur in certain regions where their body temperature is slightly cooler.

Nutritional mutations

• Mutation affecting nutrient processing → Some genetic changes can make your body less able to break down certain foods. In this case, it’s lactose, the sugar in milk.

• Lactose intolerance → Happens when the body can’t make enough lactase, the enzyme that digests lactose into glucose and galactose so it can be absorbed. Without enough lactase, lactose stays in the gut and causes bloating, gas, or diarrhea.

• Age factor → Most humans make plenty of lactase as babies because milk is the main food. After weaning, lactase production often decreases, which is why lactose intolerance is more common in adults

When do we notice a mutant gene?

• A gene’s effect doesn’t always show immediately.

• When it shows depends on:

  1. The stage of development

  2. Whether the gene product is needed at that time

  3. Internal changes in physiology as the organism ages

Lesch–Nyhan syndrome

-X linked recessive gene.

-The mutation disrupts nucleic acid metabolism, which leads to a buildup of uric acid.

-Although the mutation is present from birth, the symptoms don’t appear immediately.

• They usually start showing around 6–8 months of age

Tay Sachs Disease

-appears normal at birth but leads to developmental retardation, paralysis, blindness, and death by age 3

Duchenne muscular dystrophy (DMD)

another X-linked recessive disorder, causes progressive muscular wasting and is usually diagnosed between ages 3 and 5, often fatal by the early 20s.

Huntington disease

an autosomal dominant disorder, causes progressive brain cell death, spastic movements, intellectual deterioration, and death. Symptoms typically appear between ages 30 and 50, with a mean onset age of 38 years.

Genetic anticipation

the mutation gets bigger or worse as it passes to the next generation, causing earlier and stronger symptoms.

EX)Myotonic dystrophy (DM1)

“Blank-slate” hypothesis

human traits are shaped overwhelmingly by our social and cultural environments

Genetic determinism

animal and twin studies revealed a key role for genes in determining behavioral and intellectual traits