11- 3: Exploring Mendelian Genetics&

11-5: Linkage and Gene Maps

What are the chances?

• Do people with brown hair always have brown eyes?

• Do people with brown hair have a higher chance of having brown eyes?

• Is a round pea seed always yellow? Can a short plant have purple flowers?

INDEPENDENT ASSORTMENT

• The inheritance of one gene does not influence the inheritance of another.

• Mendel named this:

The Law of Independent Assortment• In meiosis, the chromosomes line up

randomly on the equator to be separated.

• If your parents are heterozygous for any traits, this leads to lots of possibilities!

How did Mendel figure this out?• Through experiments…• He crossed two different plants• Each was true-breeding for 2 different traits• P gen: rryy (wrinkled & green)

x RRYY (ROUND & YELLOW)

• F1 gen: RrYy• F2: a bunch of possibilities!

Segregation of Chromosomes Video

Aligning the Punnett Square• F1 gen now has: RrYy• Distribute the 1st letter of the 1st set to each

of the 2 letters in the 2nd set.• Distribute the 2nd letter of the 1st set to

each of the 2 letters in the 2nd set

R r Y y• 4 possibilities: RY, Ry, rY, ry

Patterns

A Het x Het dihybrid cross yields the typical phenotypic ratio of:

9: 3: 3: 1

Now You Try!

In humans, the gene that causes a unibrow (u) is recessive to not connected eyebrows (U); the gene for thick lips (T) is dominant over the gene for thin lips (t). If a male that is homozygous for not connected eyebrows and heterozygous for thick lips mates with a woman who has a unibrow and is heterozygous for thick lips, what is the phenotypic ratio of the offspring?

How do you set it up?• Genotypes

--Dad = UUTt --Mom = uuTt

• Possible Different Combinations of Alleles--Dad = UT or Ut --Mom = uT or ut

UT Ut

uT    

ut    

dad

mom

1. Make one column for each possible different combination of alleles from dad2. Make one row for

each possible different combination of alleles from mom

3. Fill in the boxes

U = not connected eyebrows

u = unibrow

T = thick lips

t = thin lips

Results of the Cross

UuTT UuTt

UuTt Uutt

UT Ut

uT

   

ut

   

3 not connected eyebrows & thick lips : 1 not connected eyebrows & thin lips

Phenotypic Ratio

Only include the phenotypes present in the results.

Summing It Up: Mendel’s Principles

1. Parents pass on characteristics, sexually, through genes to their offspring

2. When there are multiple alleles (appearances) for one gene, some are dominant & some are recessive

3. During formation of parental gametes, alleles are segregated into separate gametes. Each parent is then able to pass ONE allele to the child. The child therefore gets ONE allele from EACH parent

4. The chromosomes (and therefore alleles) from each parent arrange themselves independently during meiosis

11-3 Exploring Mendelian Genetics

Going Beyond Simple Dominance:1. Incomplete Dominance2. Codominance3. Multiple Alleles4. Polygenic Traits5. X- linked (or sex linked) Traits

Incomplete Dominance

• In some cases, neither allele truly dominates over the other.

• No allele is really dominant or recessive• The heterozygous genotype shows a MIX

of the two traits. • Example- Four O’Clocks

– R- gene for red flowers, W- gene for white flowers:

– RR- red, WW- white, RW- pink

Example: Snapdragons

RR = Red WW = White

All RW = Pink

Codominance• In some cases, both alleles are dominant.

– No allele is really recessive.• The heterozygous genotype shows BOTH

of the two traits. • Example- Chicken feathers

– C1- gene for black feathers, C2- gene for white feathers:

– C1C1- black, C2C2- white, C1C2- “erminette” Black and White!

Multiple Alleles• Many genes have more than just two

alleles for a trait– Remember, you can still only have 2 alleles at

a time. – It is still just ONE gene, but lots of possibilities

• Example: Alleles for rabbit fur– C-full color, dominates over cch, ch, c– cch – chinchilla, dominates over ch and c– ch- himalayan, dominates over c– c- albino, recessive to all

Practice Cross cchch x cc

• What are the phenotypes of the parents?– light gray x albino

• Fill in the Punnett Square.• What is the probability that

an offspring will be albino (albino = cc)?– 0 out of 4 = 0%

• What is the probability that an offspring will be himalayan (himalayan = chch, chc)?– 2 out of 4 = 50%

cch

c

c

ch

cchc

cchc chc

chc

Light gray

Light gray Himalayan

Himalayan

Polygenic Traits

• Many traits result from the interaction of several genes.– Multiple genes, perhaps on different

chromosomes even, – produce one phenotype

• Polygenic traits can produce a large range of phenotypes

• Examples: human skin color (at least 4 genes), human eye color, human height

Capital letters = dark; small letters = light; more dark alleles = darker!!

Gradation of Human Skin Color

Height in Humans• Range of phenotypes resulting from

polygenic trait

Human Blood Types

• Exhibit 3 special situations• Multiple alleles - A, B, O• Polygenic - one gene controls type,

another gene controls rH factor (+, -)• Codominance - A and B are codominant

but both dominate over O

What does your blood type actually mean?

Linkage

• Really it is the chromosomes that are segregated independently, not necessarily individual genes.

• Some genes are LINKED if they are on the same chromosome– Ex: you get all of the genes on chromosome

1 from your mom if you get her chr.1• Is that always the case though? What do

you know might happen?

Crossing Over• Depending on how FAR

APART genes are on chromosomes, they may be switched during meiosis– Occurs during Prophase I– Must be homologous

chromosomes

Crossing Over Video

Gene Maps

• Where actual genes are located on chromosomes.

• Discovered by a student working in molecular lab at Columbia in 1931.

• By looking at statistical inheritance patterns, you can calculate the “recombination frequency” of alleles.

• If genes are far apart, crossing over is more likely

Sex-linked Traits

• Often called X- linked traits• Trait can be dominant or recessive• Probability of inheritance is altered

because the trait is on the X chromosome• Females- XX Males- XY

Examples of X-linked traits

• Colorblindness• Hemophilia• Duchenne

Muscular

Dystrophy

Red = hemophiliac gene

Male: Has hemophilia

Female: Carries hemophilia gene