GENETICSBio 392

CHAPTER 11

Gregor Mendel• Born 1822 in Czech Republic• Austrian monk• Worked in the garden at the

monastery • Experimented with the

plants in the garden: PEAS!• Set foundation for Genetics

Flower reproduction (see Ch 24)

• Fertilization within same plant– “self-pollination”

• Stamen (male)– Anther where meiosis makes pollen – Filament stalk

• Carpel (female)– Stigma sticky, receptive part– Style stalk– Ovary where meiosis makes egg

Mendel’s Experiment

• Mendel manually cross pollinated the pea plants in his garden

• He used “true-breeding” parents– This meant that those plants always produced

offspring that looked identical to the parents

• He would cross one type with another• He called the offspring hybrids• He used seven traits in his experiments.

Generation Notation• P gen: Mendel manually fertilized one pea

plant with another to produce…• F1 gen: the 1st filial generation– He let these offspring self-pollinate to produce…

• F2 gen: the 2nd filial generation

• FYI…• For animals: you can think of these as:– P = your mom & dad, F1 = you, F2 = your kids someday– In genetic experiments, F2 does not have to be “selfed”

but it often is. This should be specified

Mendel’s 7 Pea TraitsSeed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

What happened?• When Mendel crossed one type by

the other for each trait – (eg. Green x yellow)

• only one of the traits showed up in the offspring.

• Mendel called this trait DOMINANT. • Was the other variety of the trait

gone forever?

• The other trait from the P generation showed up again in the F2 generation.• Mendel called this trait RECESSIVE.• Mendel called this the Principal of

Dominance: some alleles are dominant & some are recessive

ALLELES• These traits were passed through generations– Also called “Genes”

today– Factors had two forms:

• “Alleles” = different forms of the same gene– EX: height gene

– tall & short alleles

• Capital letters = DOMINANT alleles

• lowercase letters = recessive alleles

Simplest scenario…

• For each trait an individual has two alleles (one from each parent). In turn, each individual can only pass one or the other of its alleles to its offspring. • Mendel called this separation of

alleles SEGREGATION.

• Actual appearance (think PHoto- PHeno)• The way the trait shows itself• If T is the allele for tallness and t is the allele

for a dwarf plant: TT- tall plant Tt- tall plant tt- dwarf plant

Phenotypes

Genotypes

• The actual genes you have for a traitTT- (homozygous dominant) Tt- (heterozygous) tt- (homozygous recessive)

• The letters represent actual genes inherited (one from each parent)

Mendel + Meiosis

Probability & Genetics

probability

• What is probability?– The likelihood a particular event will occur

• Coin flipping– Chances of heads? Tails?– Two coins

• Chances of two heads? Two tails? Heads & tails?

• Rule of Multiplication– “And” Rule

• Past events do not affect future probabilities• INDEPENDENT!

11-2 Probability and Punnett Squares

• Punnett squares can be used to show the possible outcomes for a trait according to the traits of the parents

• Also show the probability of the outcome

E e

Ee x Ee

• Squares represent possible offspring• Each offspring gets one gene from

“dad” (from the top) and one gene from “Mom” (from the side)

• What % of the offspring will have free earlobes?

• What % will have attached ears?• What % will be heterozygous for the

trait?

EE

Ee

Ee ee

E

e

E

e

Applying to genetics

What are the chances?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)RRYY (ROUND & YELLOW)• F1 gen: RrYyRrYy• F2: : a ba bunchunch ofof popossissibilibilities!ties!

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 yRY, Ry, rY, ry

Patterns

A Het x Het dihybrid cross yields the typical genotypic 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 normal eyebrows and heterozygous for thick lips mates with a woman who is unibrowed and heterozygous for thick lips, what is the genotypic and phenotypic ratio of the offspring?

How do you set it up?• Dad’s phenotype: Mom’s phenotype• UUTt uuTt• Distribution of alleles• UT – Ut – UT – Ut uT – ut – uT – ut• Genotype Ratio (should add up to 16)• UuTT : UuTt : Uutt = 4 : 8 : 4• Phenotypic Ratio = 12 : 4 : 0 : 0

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. Independent Assortment2. Incomplete Dominance3. Codominance4. Multiple Alleles5. Polygenic Traits6. X- linked (or sex linked) Traits

Incomplete Dominance

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

• No allele is really dom. or rec.• 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

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– B- gene for black feathers, W- gene for white

feathers:– BB- black, WW- white, BW- “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 C chCh x cc

• What are the phenotypes of the parents?

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

that an offspring will be albino?

• What is the probability that an offspring will be 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

• http://www.athro.com/evo/gen/geframe.html

• Human eye color postulated• http://www.athro.com/evo/gen/geframe.htm

l• Human eye color calculator

Human Blood Types

• Multiple alleles, Polygenic and Codominance!• 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–Prophase I–Must be homologous

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

Extra Slides

Percentage of Blood TypesRh+ Rh -

O 38.5% 6.5%

A 34.3% 5.7%

B 8.6% 1.4%

AB 4.3% 0.7%