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

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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

More than 2 versions (alleles) for a single trait

can be completely dominant or codominant

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.

Sex-linked genes

• An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes

• Mammals like humans have an XX or XY system of inheritance

• Other organisms have other systems

Genes on the sex chromosomes are called

sex-linked genes• Some diseases on the X

chromosome:• Color blindness

• Rare in females, mild disease• Duchenne muscular dystrophy

• 1 in 3500 males in US gets it• Lack the gene for the muscle protein

dystrophin• Muscles get weaker and lose

coordination• Usually don’t live past 20s

• Hemophilia• Lack the protein to cause clotting• Don’t clot normally

Barr bodies

• In mammalian females, 1 of the 2 X chromosomes is inactivated during embryonic development

• The inactive X condenses into what is called a Barr body (we can see it under the microscope)

• If she is heterozygous for a sex-linked trait, she will be a mosaic for that trait

• Some cells have the maternal X inactivated• These cells have

the orange color• Some cells have

the paternal X inactivated• These cells have

the black color• All cells in the

ovaries have active X chromosomes

Y-linked traits Y-linked traits called holandric

inheritance. Y-chromosome is small and does not

contain many genes Deletions on y chromosome male

infertility SRY gene sex determining region

The curious case of the guevedoces

Deficient in an enzyme that converts testosterone to dihydrogen testosterone, so don’t develop male genitalia as embryos.

20.Orange and black coat color are on the X

chromosome in cats and they are codominant to each other. Tortoise shell is the codominant phenotype.

A black female (XBXB) mated with an unknown male. The kittens were:

2 tortoise shell females and 2 black males.What is the father’s genotype and phenotype?

XOY- orange

21.• Ricket’s is a dominant disorder on the X

chromosome in humans. • X = normal XR = affected by ricketsA couple wants to know their chances of having

a child born with Rickets.The wife is normal, the husband has the

disease.What are the chances of having an affected

son? An affected daughter?

0% affected son, 100% affected daughter

22.• Another couple, same disease. This

time, the wife is affected. Her father was normal. The husband is not affected. Same question: chances of an affected son? Affected daughter?

50% son, 50% daughter

23.• A tortoise shell female mated with an

unknown male. The kittens were 2 orange females, 1 tortoise shell females, 1 black male, 2 orange males.

• What is the genotype and phenotype of the father?

XOY- orange

Chromosomal mutations• In nondisjunction,

pairs of homologous chromosomes do not separate normally during meiosis

• As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy

What results…• Aneuploidy - a zygote

produced from a normal gamete and a gamete produced by nondisjunction • Offspring with this

condition have an abnormal number of a particular chromosome

What results…

• Trisomy - having 3 copies of a particular chromosome• Monosomy - having just one copy of a particular

chromosome• Polyploidy - a condition in which an organism has more

than two complete sets of chromosomes

Recent research has shown that this Chilean rodent is a tetraploid

Very rare among animals

Common in plants, some fish, some amphibians

Chromosomal breakage• Breakage of a chromosome can lead to

four types of changes in chromosome structure:• Deletion removes a chromosomal

segment

Deletion mutation• Example: retinoblastoma (eye

tumors)

Chromosomal breakage• Duplication repeats a segment

Duplication mutation – fragile X syndrome

Chromosomal breakage• Inversion reverses a segment within a

chromosome

Hemophilia A – inversion mutation patient was given

injection in buttocks

Chromosomal breakage• Translocation moves a segment from

one chromosome to another

Translocation mutation causes Burkitt’s lymphoma Tumors on

hand from cancer

Why does this happen? When would you predict these kinds of

chromosomal errors would occur?

Why?

Down syndrome• Trisomy 21 - 3 number 21

chromosomes• 1 in 700 children in US• Frequency increases with

age of mother

Trisomy 18 – Edward’s syndrome low birth weight,

mental retardation, extra fingers and toes

Trisomy of sex chromosomes• Klinefelter

syndrome is the result of an extra chromosome in a male, producing XXY individuals

• Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans

DNA is also found in mitochondria and chloroplasts.

Mitochondrial DNA is only passed from Mother to child.

How are traits inherited?

What mode(s) of inheritance would you predict for the trait of skin color? Why?

Make a list of all the possible modes of inheritance we’ve learned about

Next to each one give a short definition and an example