Introduction to Mendelian Genetics

Introduction to Mendelian Genetics

Introduction to Mendelian Genetics

The Work of Gregor Mendel

A. Genetics is the scientific study of heredity.

Every living thing has a set of characteristics inherited from its parent or parents!

• Who & Where: Austrian monk, “Father ofGenetics”, born 1822

• What: His work was important to theunderstanding of heredity. Incharge of monastery’s garden;studied traits of pea plants

B. Gregor Mendel

C. So Why Peas?

• Pea plant flowers are closed • Self-fertilizing• True-breeding• Have 7 easily visible traits called

phenotypes

Every time Mendel crossed 2 different traits, only ONE was seen in the offspring!

C. Mendel’s PrinciplesC. Mendel’s Principles

1. Principle of Dominance2. Principle of Segregation3. Principle of Independent

Assortment

1. Principle of Dominance2. Principle of Segregation3. Principle of Independent

Assortment

Back to Mendel’s Experiments…Back to Mendel’s Experiments…

If these hybrids self-pollinate..

The Next Generation

The hidden trait returns!

F1 generation: Tt x Tt Result?

Plants with different genotypes

(TT and Tt) can have the

same phenotype (“tall”).

What happened to Recessive Allele?

• Genotype: genetic makeup of organism• Phenotype: physical characteristics of an

organism

Genotype Phenotype

C c

T T

P p

D. Principle of Dominance Definition: some alleles are dominant and others are recessive. The dominant gene shows up in the phenotype when present.

Example: Smooth peas SSmooth peas S wrinkled peas wrinkled peas ss

• During sex cell formation, alleles separates from each other

• Each gamete has one allele for each trait

E. Principle of Segregation

• Each trait is controlled by a gene that is in Each trait is controlled by a gene that is in two contrasting formstwo contrasting forms

• The different forms of a gene are called The different forms of a gene are called allelesalleles..

• Each trait is controlled by a gene that is in Each trait is controlled by a gene that is in two contrasting formstwo contrasting forms

• The different forms of a gene are called The different forms of a gene are called allelesalleles..

• Homozygous: two identical alleles

Example: TT or tt or SS or ss

• Heterozygous: two different alleles for the same trait

Example: Tt or Ss

• Homozygous: two identical alleles

Example: TT or tt or SS or ss

• Heterozygous: two different alleles for the same trait

Example: Tt or Ss

II. Probability and Punnett Squares

Why used?

Punnett squares used to predict and compare genetic variations that will result from a cross

The Rules:

• Dominant traits are the 1st letter and CAPITAL

• Recessive are the 2nd letter and lowercase

B. Practice 1) If a homozygous tall person was crossed with a homozygous short person, what are probably offspring?

Tall= Tshort= t

T T

t

t

T

T

T

T

t

t

t

t

Punnett Square2) Cross two heterozygous tall parents.

Tall= Tshort= t T

TT T T

Tt t

t

t

tt

What is the genotype ratio?

1 TT: 2 Tt: 1 tt1 TT: 2 Tt: 1 ttWhat is the phenotype ratio?3 Tall: 1 short3 Tall: 1 short

Punnett Square

3) The long-eared allele (L) is dominant to the short-eared allele (l). Cross a homozygous long ear with a homozygous short-ear.

Cross the F1 generation and give the F2 results.

III. Independent Assortment

A. Mendel discovered that genes for different traits segregate independently during gamete formation

Ex. Wrinkled/Smooth and Yellow/Green peas

III. Independent Assortment

A. Mendel discovered that genes for different traits segregate independently during gamete formation

Ex. Wrinkled/Smooth and Yellow/Green peas

B. Dihybrid Cross: two traits being crossed at the same time

Here is a heterozygous tall, heterozygous purple plant:

T t T t P pP p

Tall and Purple are dominant to short and white.

Example: Example: CrossCross two heterozygous tall, two heterozygous tall,heterozygous purpleheterozygous purple pea plants.pea plants. T t T t P p P p x T t x T t P pP p

There are four possible gametes each parent can make..

Tall and Purple are dominant to short and white.

Example: Example: CrossCross two heterozygous tall, two heterozygous tall,heterozygous purpleheterozygous purple pea plants.pea plants. T t T t P p P p x T t x T t P pP p

There are four possible gametes each parent can make..

T TP tp tP

T

p

P

Tp

tP

pt

TTPp

TTPP TTPp TtPP TtPp

TtPpTTpp Ttpp

TtPP TtPp ttPP ttPp

TtPp Ttpp ttPp ttpp

MOTHERF

AT

HE

R

Results in fractions?

Phenotypes:

Tall, Purple?

Tall, White?

Short, Purple?

Short, White?

Results in fractions?

Phenotypes:

Tall, Purple?

Tall, White?

Short, Purple?

Short, White?

9/16

3/16

3/16

1/16

Ratio from heterozygousheterozygous dihybrid cross is ALWAYS

9: 3: 3: 1

Alleles assort independently.

Use FOIL to set up these examples:

FfPp:

SSTt:

DdRR:

FP Fp fP fp

ST ST St St

DR DR dR dR

IV. Beyond Pure Dominance….

Some alleles are not simply dominant orrecessive..

A. A. Incomplete dominance:Incomplete dominance:Alleles are expressed as a blend.Each allele has a capital letter.

Red= R Yellow= Y

Red=RWhite=W

1. Cross a red flower with a white flower, showing incomplete dominance.

R R

W

W

R R

R R

W W

W W

Genotype: 100% RWPhenotype: PINK!

D. Co-dominance• Both traits dominate, seen separatelyseen separately!Red Horse White Horse

Give you ROAN!

1. Example of Codominant Problem

Red feathers are codominant to white feathers in chickens.

CR= red

CW= white

Cross a homozygous Red with a homozygous white feathered chicken.

CR CR

CW

CW

CR CW CR CW

CR CW CR CW

PHENOTYPE:100%Red and white mixed feathers

CR CWGENOTYPE:100%

• One trait, many allele options!

• But remember: an individual cannot inherit more than two actual alleles, even if more than two possible alleles exist.

Example: Blood type A, B, AB, O!

Blood Type Problem I

• Cross a homozygous Type A with a heterozygous Type B. What are the possible phenotypes of offspring?

IB

IA

IB

i

IA

IA

IA i

Phenotypes:50% Type AB50% Type A

Blood Type Problem II

• Cross a heterozygous Type A man with a heterozygous Type B woman. Is it possible for them to have an O child?

IB

i

IB

i

IA

IA

IA i

Phenotypes:25% Type AB25% Type A25% Type B25% Type O

IB i

i i

Blood Type Problem III

• Cross a heterozygous Rh+ man with a Rh- woman. What are the possible phenotypes of offspring?

Rh-

Rh-

Rh-

Rh-

Rh+

Rh+

Rh+ Rh-

Phenotypes:50% Type +50% Type -

Rh- Rh-

Rh- Rh-

Rabbits have 4 basiccolors (alleles!)

• brown• chinchilla or grey

• It is recessive to brown.

• himalayan or white with black tips.

• It is recessive to both brown and chinchilla.

• albino• It is recessive to all.

AIbinoHimalayan

ChinchillaFull color

D. Polygenic Traits• Traits produced by many genes with

many alleles• Most human traits are polygenic• Most variety of expression• There are 3 genes that contribute to

skin color.. And many alleles for each gene!

More examples:

• Height• Weight• Intelligence• Eye color

V. Sex Determination

In humans, the X and Y chromosomes control the sex of offspring.

Outcome is always 50% chance of a male, and 50% chance of a female

Sex-linked traits• Traits controlled by genes on the sex

chromosomes are called sex-linked.

• Alleles for sex-linked traits are written as superscripts on the X chromosomes only.

• Traits controlled by genes on the sex chromosomes are called sex-linked.

• Alleles for sex-linked traits are written as superscripts on the X chromosomes only.

XR XR Xr yExample: Red eyes in fruit flies found in femalesMales tend to have white eyes, which is recessive.

• X and Y sex chromosomes are non-homologous

• Any allele on the X chromosome will NOT be masked by a matching allele on the Y chromosome.

Why are sex-linked disorders more common in males than in females?

• Males have just one X chromosome containing an allele. So all X-linked alleles are automatically expressed in males, even if they are recessive.

Color blindnessColor blindnessDuchenne Muscular DystrophyDuchenne Muscular DystrophyHemophiliaHemophilia

C. Examples of Sex-LinkedC. Examples of Sex-Linked

Frank and Awilda at BreakfastFrank: Are you sure you want to wear that new shirt to work today? A green and red shirt like that would be better for Christmas, not for St. Patrick's Day.Awilda: Oh no! Not again! I hate being color blind! I really thought this shirt was just different shades of green. Where's the red?

At Dinner That NightAwilda: We should try to find a way to make sure we only have sons, no daughters. I don't want to have any daughters who might be color blind and have so many problems like I do. Color blindness wouldn't matter so much for a boy.

Frank: Remember, the doctor said that, since I'm not color blind, none of our daughters would be color blind, only our sons.

Awilda: That doesn't make any sense. Our daughters should be color blind like me and our sons should be normal like you.

Frank: No, the doctor said the gene for color blindness is on the X chromosome, so only our sons will inherit your colorblindness.

Awilda: I don't agree. Girls have more X chromosomes than boys, so girls should be more likely to be color blind.

Help Frank to explain to Awilda why the doctor was right by answering the following questions.

1. What are the genotypes of Awilda and Frank?

(Since the allele for color blindness is recessive and located on the X chromosome, use the symbol Xc for an X chromosome with the allele for color blindness and XC for an X chromosome with the normal allele.)

Awilda: Frank: Xc Xc XC y

2. Draw the Punnett square for this couple and their children. In this Punnett Square, circle each daughter and use arrows to indicate any colorblind offspring.

X X

y

X

c

C

C = normal visionc = colorblind

c

XC Xc XC Xc

Xc y Xc y

3. Write an explanation to help Awilda understand why their daughters will not be colorblind like their mother.

4. Explain why their sons will be colorblind even though their father has normal vision.

5. Explain why having two X chromosomes decreases a person’s risk of color blindness, instead of increasing their risk, as Awilda fears.

Practice ProblemsPractice Problems

Hemophilia is an X-linked Hemophilia is an X-linked recessiverecessive disease. Cross a heterozygous disease. Cross a heterozygous female with a normal male.female with a normal male.

Duchenne Muscular Dystrophy is an Duchenne Muscular Dystrophy is an X-linked X-linked recessiverecessive disease. Cross a disease. Cross a heterozygous female with a normal heterozygous female with a normal male. male.

Examples of Sex-linked DiseasesExamples of Sex-linked Diseases

ColorblindnessColorblindness

D. Sex-Limited Traits• A few traits are not caused by genes on the X

or the Y chromosome but still occur in only one sex of animals– Examples

• Antlers in deer- only bucks have antlers• Milk yield in bovines is a trait expressed by only cows (females) • Eggs in chickens

E. Sex-Influenced• Some traits are sex-influenced

because of genes that interact with a substance (like hormones) that is not produced equally in males and females– Example: early pattern baldness

Baldness Sample Problem

• Baldness is a dominant trait. Heterozygous men are bald, BUT heterozygous women have all hair.

• Cross a Heterozygous woman with a normal hair male. Bb x bb

b

b

bBGenotype - Phenotype

If all girls?If all boys?

B b b b

B b b b

Human Genetic DisordersHuman Genetic Disorders

Symptoms: learning difficulties, mental retardation, a characteristic facial appearance, and poor muscle tone

Detection/Frequency?

1 in 1000 live born infants

Mode of Inheritance/Chromosome

Chromosome 21, nondisjunction

Treatment Physical therapy for muscle weakness, heart is checked regularly for problems, educational therapy

Prognosis May have shortened life span

Down Syndrome

Marfan SyndromeSymptoms: Myopia, retinal detachment, bone

overgrowth and loose joints, may have long thin arms and legs, bent chest inwards or outwards


occurring 1 in 10,000 to 20,000 individuals


Autosomal dominant, Chromosome 15

Treatment Surgery to correct skeletal problems, sight issues fixed with glasses, must avoid contact sports

Red-Green Colorblindness

Symptoms:



Treatment

Prognosis

Retinoblastoma

Symptoms:



Treatment

Prognosis

Albinism

Symptoms:



Treatment

Prognosis

Duchenne Muscular Dystrophy

Symptoms:



Treatment

Prognosis

Turner SyndromeSymptoms:



Treatment

Prognosis

Dwarfism (Achondroplasia)

Symptoms:



Treatment

Prognosis

HemophiliaSymptoms:



Treatment

Prognosis

Huntington’s DiseaseSymptoms:



Treatment

Prognosis

Tay-Sach’sSymptoms:



Treatment

Prognosis

Klinefelter’sSymptoms:



Treatment

Prognosis

Cystic FibrosisSymptoms:



Treatment

Prognosis

Sickle Cell AnemiaSymptoms:



Treatment

Prognosis

Phenylketonuria (PKU)Symptoms: causes increase of phenylalanine in blood -

results in mental retardation, heart problems, small head size (microcephaly) and developmental delay


1 in 10,000 to 1 in 15,000 newborn babies


Treatment Limiting dietary intake of phenylalanine

Prognosis

Symptoms:



Treatment

Prognosis

Symptoms:



Treatment

Prognosis

11.5 Linkage & Gene Maps

Thomas Hunt Morgan, 1910Research fruit fliesFound 50+ Drosophilia genesMany of them “linked” togetherAll the genes from one group were inherited together

Chromosomes assort independently, not the genes

How did Mendel miss this linkage?

By pure luck, the 6 genes he looked at were on different chromosomes

Gene MapsCrossing-over sometimes separates genes on the same chromosomes onto homologous chromosomes.

– Occasionally separate and exchange linked genes and produce new combinations

The farther apart two genes are, the more likely they are to be separated by a crossover in meiosis.Alfred Sturtevant created a gene map showing the locations of each known gene on one of the Drosophila chromosomes

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Introduction to Mendelian Genetics