Mendelian GeneticsMendelian Genetics

GeneticsGeneticsthe branch of Biology that deals withthe branch of Biology that deals with

INHERITANCEINHERITANCE(the passing of traits from parents to offspring)(the passing of traits from parents to offspring)

Gregor MendelGregor Mendel~Austrian monk who studied pea plants in the ~Austrian monk who studied pea plants in the

1850’s1850’s~theorized that characteristics are inherited as ~theorized that characteristics are inherited as

a result of the transmission of “factors” from a result of the transmission of “factors” from parents to offspring (he didn’t know about parents to offspring (he didn’t know about genes & chromosomes!)genes & chromosomes!)

Mendel used pea plants b/c he Mendel used pea plants b/c he noticed contrasting traits:noticed contrasting traits:

Tall plants vs. short plantsTall plants vs. short plants

Green pods vs. yellow podsGreen pods vs. yellow pods

Wrinkled seeds vs. Wrinkled seeds vs.

smooth seedssmooth seeds

““Brother Mendel, we grow tired of peas”Brother Mendel, we grow tired of peas”

~his work wasn’t valued until the ~his work wasn’t valued until the early 1900’s when scientists began early 1900’s when scientists began observing contrasting traits in observing contrasting traits in Drosophila (fruit flies)Drosophila (fruit flies)

and when better and when better microscopes led microscopes led to the discoveryto the discoveryof chromosomesof chromosomes

Drosophila Drosophila have short life have short life cycles and few chromosomes cycles and few chromosomes

and therefore are easy to and therefore are easy to study in geneticsstudy in genetics

Mendel is considered to be Mendel is considered to be the “Father of Geneticsthe “Father of Genetics””

The garden at Mendel’s Monastery

II. The Gene-ChromosomeTheory-II. The Gene-ChromosomeTheory-

gene- gene-

allele- another word for geneallele- another word for gene ex) ex)

2 alleles/genes for flower 2 alleles/genes for flower color: one white, one color: one white, one

purplepurple

loci-loci-

chromosomes contain genes which have all the instructions for the traits of an organism

-the unit of hereditary material found on chromosomes

- genes are made of DNA

Latin for “place”; the place/position on a chromosome where you find a gene/allele

Add drosophila loci picture

Loci on fruit fly chromosome

Homologous chromosomes:Homologous chromosomes:

pairs of chromosomes that have genes

for the same traits. You have 2 genes

for every trait and they’re located on

homologous chromosomes.

XXXX (double stranded homologs) II (single stranded homologs)

III.Major Genetic Concepts

III.Dominant Allele

represented by a _________letterexample: tallness=______

Recessive Allele-

represented by a _________letterexample: lack of tallness

(shortness)=_____

-the allele (gene) that is expressed if present

-the “stronger” allele

capital

T

the gene that isn’t expressed if dominant is present. - the “weaker”, rarer allele

lower case

t

Genotype-Genotype-

Homozygous-Homozygous-

the genes (letters) of a trait described as homozygous or heterozygous

ex) Dd; heterozygousDD; homozygous dominant

dd; homozygous recessive

also called purewhen both genes are the same, either dominant or recessive.

ex) TT, tt, DD, dd

Heterozygous- also called ________

Phenotype-

hybrid-when both genes are different; one dominant, one recessive

ex) Dd, Tt

the physical appearance of a trait

ex) tall, short, curly, straight

(the genotype is the musical score and the phenotype is the music we hear)

Ataxia video link

~Mendel wondered how he could cross 2 tall pea plants and end up w/some tall & some short

~he theorized that the recessive trait can disappear in 1 generation and show up in the next generation as follows:

Genes T,t are SEGREGATED or

separated during MEIOSIS

Tt Tt

T t

TtTT

tT

ttTt

Parents: x

meiosis

fertilization

F1 possibilities

All of the genes are RECOMBINED RANDOMLY during fertilization(this diagram is a graphical representation of a Punnet Square)

Law of Segregation and Recombination:

Mendel explained that “factors” which occur in pairs are separated from each other during gamete formation and recombined at fertilization.

Law of Independent Assortment:

Mendel concluded that different traits are inherited independently of one another (genes for different traits are separated & distributed to gametes independently of one another when they are on different chromosomes).

He learned this after doing dihybrid crosses which examine 2 traits (like pea color & pea plant height); yellow peas aren’t always on tall plants…More on dihybrids to come…..

PUNNETT SQUARE-Predicts possible outcomes of a genetic cross

T OR t

T

OR

t

T

T

t

t

T T

t t

Segregation-Only 1 gene from each parent is passed alongdue to meiosis

Filling in the squares represents fertilization Whichrecombines genes

Results: 75% dominant25% recessive

Problems:1.If a heterozygous freckled person marries a non-freckled person, what is the chance that they will have a non-freckled baby? Freckles (F)

no freckles (f)F f

f

f

Ff

Ff

ff

ff

50% chance of Non-freckled (ff) baby

2. The gene for brown eyes (B) is dominant over the gene for blue eyes (b). Show the results of a cross between a hybrid brown-eyed woman and a hybrid brown-eyed man.

B=brownb=blue

B b

B

b

BB Bb

Bb bb

phenotype genotype

75% brown 50% Bb25% blue 25% BB

25%bb

3.Normal skin pigmentation (A) is dominant over albino (a). Show the cross of an albino man with a woman who is homozygous normal.

A= normala=albino

A A

a

a

Aa Aa

Aa Aa

phenotype genotype

100% 100% normal heterozygous

Ff freckled

tt non taster

Bb brownhh no disease

+,+ has blood factormm light skin

CC color vision

Ff freckeld

TT Taster

bb blond

hh no disease

+,- has blood factor

mm light skin

Cc color vision

ff no frecklestt non tasterBB brown hairHh has disease

-,- doesn’t have blood factorMM dark skincc color blind

Can’t tell if he was Ff or ffyesFF

Can’t tell if parents were Tt or tt

Can’t tell if parents were Bb or bb

Can’t tell if parents were Cc or cc

yes

brown

dark

Intro to dihybrid crosses (not in note pkt)

As Mendel continued his study of peas, he noticed that different traits (pea color, pea shape) were inherited independently of one another.

When he crossed yellow x yellow, the F2

smooth smooth

generation had : yellow, yellow, green, green smooth wrinkled smooth wrinkled

We know now, that during meiosis, genes for different traits are separated and distributed to gametes independently of one another. (when they’re on different chromosomes).

Genes located on the same chromosome do NOT sort independently (aka gene linkage). Breeding experiments that examine 2 different traits are called dihybrid crosses

Dihybrid Crosses

In guinea pigs, black coat color (B) is dominant to albino (b); coarse coat (R) is is dominant to smooth (r) . Two animals are selected for breeding. Their genotypes are BBRR and bbrr. Give the results of the following including expected genotypes and phenotypes of:

a) the F1 generationb) the F2 generationc) offspring produced from a cross of an F1pig with one having genotype BBRr

a) BBRR x bbrr pure parents

BR br

BbRr

gametes

F1 generation

100% black course

b)F2: BbRr x BbRr

BR BR Br Br possible bR bR gametes br br

BBRRBBRRBBRrBBRr BbRRBbRR BbRrBbRr

BBRrBBRr BBrrBBrr BbRrBbRr BbrrBbrr

BbRRBbRR BbRrBbRr bbRRbbRR bbRrbbRr

BbRrBbRr BbrrBbrr bbRrbbRr bbrrbbrr

BR Br bR br

BR

Br

bR

br

Phenotype results9 black coarse3 black smooth3 albino coarse1albino smooth

9:3:3:1

KeyB=blackb=albinoR=Coarser=smooth

c)BbRr x BBRr

BR BRBr Br possible bR gametesbr

BBRRBBRR BBRrBBRr

BBRrBBRr BBrrBBrr

BbRRBbRR BbRrBbRr

BbRrBbRr BbrrBbrr

BR Br

BR

Br

bR

br

Phenotype results 6 black coarse 2 black smooth

Statistically:If x = the number of traits in the cross

2x= # phenotypes3x= # genotypes4x= # individuals

MonohybridResults: 2 phenotypes Tt x Tt

3 genotypes 4 individuals

Dihybrid TtBb x TtBbResults: (2)2= 4 phenotypes 9:3:3:1

(3)2= 9 genotypes (4)2= 16 individuals

Trihybrid TtBbRr x TtBbRrResults: (2)3= 8 phenotypes

(3)3= 27 genotypes (4)3= 64 individuals

Dihybrid Problems….answers

1. L= long G = grayl = vestigal g = ebony

lg

LG

F1 llgg x LLGG

LlGg

100% LlGg; long, gray

F2 LlGg x LlGg

LG Lg lG lg

LG

Lg

lG

lg

LLGG LLGg LlGG LlGg

LLGg LLgg LlGg Llgg

LlGG LlGg llGG llGg

LlGg Llgg llGg llgg

9 long gray

3 long ebony

3 vest. gray

1 vest ebony

2. R= red T = tallr = yellow t = dwarf

rT

Rt

F1 RRtt x rrTT

RrTt

100% RrTt; tall, red

F2 RrTt x RrTt

RT Rt rT rt

RT

Rt

rT

rt

RRTT RRTt RrTT RrTt

RRTt RRtt RrTt Rrtt

RrTT RrTt rrTT rrTt

RrTt Rrtt rrTt rrtt

9 red tall3 red

short3 yellow

tall1 yellow

short

3. T = taste N = normal pigmentt = non taste n = albino

Tn

tn

tN tn

Mom: ttNn x Dad: Ttnn

TtNn Ttnn

ttNn ttnn

25% taster, normal25% taster, albino25% non-taster, normal25% non-taster, albino

from her father from his mother

4. B= bark E = erect earsb = silent e = droopy ears

EeBb x eebb

EB Eb eB eb

eb EeBb Eebb eeBb eebb

25% each: erect

barking

erect silentdrooping

barking

drooping silent

Intermediate InheritanceMany genes do not follow the patterns of dominance:may produce a trait between the 2 parents-

INCOMPLETE DOMINANCE (blending)

red flowers + white flowers = pink flowers “R” + “W” = “RW”

OR

May produce a trait that expresses 2 dominant alleles at the same time :CODOMINANCE

red fur + white fur = red & white fur CR+ CW = CRCW

Ex) blood type AB: IAIB

Test Cross:Consider a phenotypically tall organism. What is it’s genotype? Is it TT or Tt? To answer this question, you would do a test cross:

definition:

*By observing the phenotypes of the offspring, we can trace back to the genotype of the parent:

If the genotype If the genotype of the parent was of the parent washomozygous (TT), heterozygous (Tt),

all the offspring are tall: 50% of the offspring

are tall and 50% are short:

when an organism showing the dominant trait is crossed with a pure recessive to determine if that dominant organism is homozygous or heterozygous

TtTt TtTt

TtTt TtTt

TtTt tttt

TtTt tttt

T TT t

t

t

t

t

Problem:

1. A cattle rancher buys a black bull which is supposed to be a purebred. Black coat is dominant to red. The new owner decides to be sure he has gotten a good deal by mating the bull with several red cows. Such crosses are called test crosses. If the farmer really got a good deal, what color calves should be born?

BbBb BbBb

BbBb BbBb

B= blackb= red

B B

b

b They should all be black

Multiple Alleles:~when there are more than 2 (“multiple”) alleles

for a trait~example: Human blood groups

have 3 alleles: IA where I is dominantIB and i is recessivei

~the possible combinations of these 3 blood alleles are as follows:

Blood type genotypePhenotype

A

B

AB

O

IAIA or IAi

IBIB or IBi

IAIB

ii

Problems:1.A couple preparing for marriage have their blood typed

along with the other required blood tests. Both are type AB. They ask what types of blood their children may have. What will you tell them?

IA IB

IA

IB

IAIA IAIB

IAIB IBIB

Phenotype Genotype 50% AB IAIB

25% A IAIA

25% B IBIB

2. A type A person marries a type A person. Their firstborn has type O blood. What are the genotypes of the parents and the child?

parent ___ ___ parent ___ ___ child ___ ___

IA i IA i i i

IA i

IA

i

IA IA IAi

IAi ii

Each parent must have a recessive gene to give to

the child

3. A wealthy, elderly couple die together in an accident. Soon a man shows up to claim their fortune claiming that he is their long lost son. Other relatives dispute the claim. Hospital records show that the deceased couple were blood types AB and O. The person claiming to be their son was type O. Do you think the man is an imposter?

IA i IBi

IA i IBi

IA IB

i

i

50% A

50% B

He was an imposter!

Sex Determination:~there are 2 types of chromosomes: sex chromosomes (1 pair)

autosomes (all other pairs)

~in each diploid human cell this looks like: sex chromosomes 1 pair autosomes 22 pairs

23 homologous pairs or _____chromosomes~The genotype XX represents a _______________

~The genotype XY represents a _______________

~The sex of an individual is determined by the _______________ at the time of________________________.

~The human egg contains only ______ chromosomes while the human sperm can contian either ______ or _______ chromosomes.

XXXX XXXX

XYXY XYXY

X X

X

Y

Every time a man and woman have a child there is a ________ chance it will be a boy or girl.

50 / 50

46female

male

spermfertilization

XX Y

Sex Linkage:~sex-linked traits are caused by genes found on the X chromosome~sex-linked traits are recessive~since they are recessive, fewer females are afflicted with these traits because they have another X chromosome which is dominantly normal. Males have only one X chromosome so when they have a sex-linked gene, they’ll display the trait.~the genotype of a sex-linked trait is represented as_________~females with one gene for the trait ( X X ) are called_____________~examples of sex-linked traits: hemophilia, color- blindness

X or X (can also be represented as Xh, where h is the trait)

carriers

1. A man normal for blood clotting marries a woman who is a carrier for hemophilia. What are the chances they will have a child with hemophilia?

25% chance of a child with hemophilia:

25% normal female (XX)25% carrier female (XX)25% normal male (XY)25% hemophila male (XY)

X Y

X

X

XX XY

or XHXH XHY

XX XY

or XHXh XhY

2.What is the probability that a woman with normal vision who marries a

colorblind man will have a colorblind child?

0% chance:50% carrier females50% normal males

X X

X

Y

XX XXOr XHXh XHXh

XY XYXHY XHY

3. A man with a normal vision and a woman with normal vision have 3 sons. Two of the

sons have normal color vision and one of them is color-blind. What are the probable genotypes of the parents?

XY and XX ;The gene for color blindness must be hidden/carried by mother to be passed on

X Y

X

X

XX XYor XhXH XhY

XX XYXHXH XHY

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

The Y chromosome

Pedigree charts: used to show he presence or absence of a certain trait in families through

several generations.**they’re kind of like a “genetic family tree”

Bb

bbbb

BbBb

bb

Bb

1

3

X Y

X

X

XX XY

XX XY

Human PedigreeA diagram showing the transmission of a trait through several generations of

a family is called a pedigree. In Figure 1, generation 1 is made up of grandparents, generation II is their children, and generation III is their grandchildren.

1. Study the pedigree diagram and the key in Figure 1 to learn the symbols.

1. How many generations are shown?_____

2. How many men are shown?_____ women?_____

3. How many women have cystic fibrosis?_____

4. How many men have cystic fibrosis?_____

5. How many marriages are shown?_____

6. How many single women are in the family?_____

7. How many children are there?_____

8. How many grandchildren are there?_____

9. Do you think the gene for cystic fibrosis is dominant or recessive?_____________________________________________________

3

5

1

4

2

5 9

5

4

Recessive b/c it remains “hidden” in several people

Use A to represent the allele for the ability to taste PTC, a dominant allele. Use aa for the PTC nontaster, who exhibits the recessive trait.

AaaaAaaa

aaaaaaaaAaaaaaaa

aaaaAaAa