Warm up Match the items on the left with one item on

the right1. HH A. heterozygous2. Curly hair B. homozygous3. Hh C. phenotype4. Genotype D. tt

Helpful

Crash Course Biology Hank Green

Bozeman Biology

Paul Anderson

Inheritance

Ch. 11

Main Topics Gregor Mendel’s work Mendel’s Laws Dominant/recessive Heterozygous/homozygous Alleles Codominance and incomplete dominance Epistasis, Pleiotropy, Multifactorial

Inheritance, Polygenic Traits

The father of genetics

Gregor Mendel is considered the Father of Genetics

Born in 1822 Studied math &

physics at an Austrian university

He was the first person to study how traits are passed along from one generation to the next.

He did his work with the pea plant

Who’s your

daddy?

Mendel’s Garden

Analyzed observable traits of peas growing in his monastery garden.

Mendel’s Garden

Eight years & 20 volumes of data and analysis on 7 distinctive traits

Published in 1865

Why peas?

The garden pea was a good choice for a variety of reasons. The garden pea: is easy to raise produces large numbers of offspring reproduces quickly has flowers which are self fertilizing

but can be easily crossed to other varieties

Experimental Approach

Can also be cross-fertilized by human manipulation

Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits

Allele for purple flowers

Homologouspair ofchromosomes

Allele for white flowers

Locus for flower-color gene

Mendel's Theory of Segregation

Diploid organisms inherit two genes per trait

Each gene segregates from the other during meiosis so that each gamete will receive only one gene per trait

How can the Chances of an Offspring’s Traits be Determined?

The chance of an offspring showing a certain trait can be determined by using the Punnett square.

The table contains spaces for the parent’s gametes and the possible offspring from that mating.

The alleles are represented by their letters. Genes come in pairs and must be separated during

gamete formation. These gametes (letter) of each pair are placed in

each of the outside spaces. They are then combined to form the possible

offspring.

Punnett Square: Bb X Bb

bbBbb

BbBBB

bBGametes

Monohybrid Crosses

Mendel's first experiments

One trait Monohybrid crosses

have two parents that are true-breeding for contrasting forms of a trait.

All the offspring from the first cross showed only 1 form of the trait

This trait seemed “stronger” so he called it DOMINANT

When he crossed the offspring from the first cross, the other form of the trait reappeared, but only 1/4 of the time

This trait seemed “weaker” so he called it recessive

Predicting the OutcomeWhy does one form of the trait

disappear in the first generation (F1 ),

only to show up in the second generation (F2 )??

Artificial selection: populations could evolve (i.e. change) if members show variation in heritable traits

Variations that improved survival chances in the wild would be more common in each generation

This idea is known as natural selection

Prevailing Theories

Mendel’s Experiments Natural selection did not fit with

prevailing view of inheritance-blending Blending would produce uniform

populations; such populations could not evolve

Mendel’s Experiments Many observations did not fit

blending A white horse and a black horse did

not produce only gray horses

Test (Back) Crosses To support his concept of

segregation, Mendel crossed F1 plants (Pp) BACK with homozygous recessives (pp)

What ratio would Mendel have gotten?He didn’t know the letter

combination of the F1 plants. The test (back) cross allowed him to figure it out

Dominant phenotype,unknown genotype:

PP or Pp?

If PP,then all offspring

purple:

p p

P

P

Pp Pp

Pp Pp

If Pp,then 1

2 offspring purpleand 1

2 offspring white:

p p

P

Ppp pp

Pp Pp

Recessive phenotype,known genotype:

pp

His back crossed supported his idea of 2 “factors” for each individual, and the idea that those “factors” are segregated

Dihybrid Crosses

Mendel also performed experiments involving two traits

Predicting the OutcomeWhat is the predicted

PHENOTYPIC ratio

and the predicted

GENOTYPIC ratio that Mendel saw?

Predicting the Outcome The F2 results showed

9/16 were tall and purple-flowered and 1/16 were dwarf and white-flowered-as were the original parents; however, there were 3/16 each of two new combinations: dwarf purple-flowered and tall white-flowered.

OutcomesMonohybrid crosses

Both parents HETEROZYGOUS3:1 phenotype

Dihybrid crossesBoth parents HETEROZYGOUS

9:3:3:1 phenotype

Theory of Independent Assortment

Each gene of a pair tends to assort into gametes independently of other gene pairs on non-homologous chromosomes

Theory in Modern Form

Genes located on non-homologous chromosomes segregate independently of each other

Practice with your neighbor

For the following questions Work with your neighbor to answer

the question. Answer the multiple choice

questionthen,

Use your notes to determine which one of Mendel’s principles it demonstrates

1. A father carries 2 alleles for the gene for widow’s peak. He

carries one dominant allele and one recessive allele. His

gametes willa. All contain the dominant alleleb. All contain the recessive allelec. ½ will get the dominant allele and ½ will get

the recessive alleled. Each gamete will get both the dominant and

the recessive allele

Which principle does question number one best

demonstrate? Principle of

Segregation

The dominant allele goes to one gamete and the recessive allele goes to another gamete

2. A mother that is homozygous dominant for bushy eyebrows (BB) and heterozygous for round ears (Rr). The gametes she can make

will a. All have a B and a R in themb. ½ will have a B and ½ will have a R or

a r in themc. ½ will have a B and a R and ½ will

have b and rd. ½ will have B and R and ½ will have B

and r

What principle does number 2 demonstrate?

The Principle of Independent Assortment

All gametes will have a B, since mom only has B.

The big B can be with the big R or the big B can be with the little r.

3. In meiosis, a diploid cell divides twice to form 4 haploid

gametes. Each gamete contains:

a. A complete set of DNA identical to the parents

b. A ½ set of DNA, with just one copy of each chromosome

c. Homologous pairs of chromosomesd. Multiple copies of chromosomes,

depending on which ones moved during meiosis

Which one of Mendel’s Principles does number 3

demonstrate? Principle of Segregation

All the homologous pairs of chromosomes separate so that there is just one of each pair in each gamete.

4. When Mendel crossed a true breeding green pea plant (GG) with a true breeding yellow pea plant (gg), the offspring plants

werea. All greenb. All yellowc. ½ green and ½ yellowd. Green and yellow mixed

Which one of Mendel’s principles does number 4

demonstrate? Principle of Complete Dominance

All offspring were Gg, and the dominant allele (G) masked the recessive allele (g)

5. Mendel wanted to know if the color for pea seeds was linked to the shape of the pea seeds. He crossed a green, wrinkled seed

plant (Ggrr) with a yellow, smooth seed (ggRr) plant. The offspring

produced were:a. All green and wrinkledb. All yellow and wrinkledc. All green and smoothd. All yellow and smoothe. Some of each of the above

Which one of Mendel’s Principles does number 5

demonstrate? Principle of Independent

Assortment

The green trait can go with the smooth or the wrinkled trait

The yellow trait can go with the smooth or the wrinkled trait

Mendel’s Work

The work that Mendel did helped explain patterns of inheritance in eukaryotes.

But Mendel worked with traits that had a clear dominant/recessive pattern.

Also, the traits he worked with were all controlled by a single gene.

Different Patterns of Inheritance As we now know,

many traits do not follow Mendelian Inheritance patterns.

Degrees of Dominance Complete Dominance - BB and Bb =

same phenotype Incomplete Dominance - Bb has in-

between phenotype Codominance - Bb has both B and b

phenotype

Co-dominance When both

alleles are expressed equally in the heterozygous individual.

A and B blood type alleles are co-dominant, because a person with AB genotype will have both A and B blood proteins.

Black and orange color in cats are co-dominant, because a heterozygous female will have both orange and black hair.

Incomplete Dominance Both alleles are

blended together in the heterozygous individual.

Dominant allele cannot completely mask the expression of another

Multiple Alleles

traits controlled by more that one gene (2 alleles) and so they have many different possible phenotypes.

These alleles can show dominant/recessive patterns or codominant patterns.

Blood TypesGenotype of offspring

Phenotype of offspring

A

iAiB AB

iAi A

iAiA

iBiB B

iBi Bii o

Rh factorRh factor Possible genotypes

Rh+

Rh-

+/+ or +/-

-/-

So far we’ve only looked at how a single gene pair affects phenotype

More often - multiple genes involved 2 primary cases:

1. 2 or more genes affect a single trait 2. 1 gene affects the phenotype of

another gene

Epistasis (standing upon)- 2 or more genes affect a single

trait Labs can be black, yellow, or chocolate

Black is dominant to chocolate

BB and Bb = black bb = chocolate

AND - another gene P codes for whether or not any pigment is put into the hair

PP and Pp = hair has pigment and dog will be black (BB or Bb) or brown (bb)

pp = no hair pigment and dog will be yellow, regardless of the “b” alleles

So in this case, the P gene “stands upon” the B gene

P is epistatic to B We don’t get the classic 9:3:3:1 but some

other version of it

Pleiotropy A single gene can

have multiple effects on phenotype

e.g. pleiotropic alleles --> multiple symptoms of sickle cell anemia (pain, jaundice, infections, fatigue, etc)

Polygenic Inheritance 2 or more genes affect a single

phenotypic trait Eye color, skin color, height

Skin color is controlled by at least 3 separate gene pairs

Genotype AABBCC would be very dark skin

Genotype aabbcc would be very light skin Any other combination would be

intermediate

And, of course, skin color is also influenced by your environment - multifactorial inheritance

X-linked traits

genes found on the X chromosome. show different inheritance patterns

in men than in women. X-linked traits may show

dominant/recessive or codominant patterns.

X-linked traits Women have 2 X-

chromosomes, men have an X and a Y.

For women to express a recessive phenotype, they must inherit 2 X-chromosomes, both with the recessive allele.

For men to express the recessive phenotype, they need only 1 recessive X

DNA in organelles DNA is also found in

mitochondria and chloroplasts.

Mitochondrial DNA is only passed from Mother to child.