genetics study of inheritance, the stability and variance of inheritance patterns gregor mendel,...

Genetics• Study of inheritance, the

stability and variance of inheritance patterns

• Gregor Mendel, “The Father of Genetics”

• Worked with the garden pea plant using cross pollination

• What would happen if we all had the exact same DNA sequences? What if we had extremely divergent DNA sequences?

• What determines which genes we inherit from our mother and father?

Genetic TermsGene-basic unit of inheritance, arranged in a linear

sequences on a chromosomesAllele-alternative forms of a gene; alleles contain genetic

information that is expressed as traits. Examples of traits are flower color, seed shape, plant height

etc.Each trait can have a different version (allele); flower color

can be purple or white, we use one letter for the trait flower color, but change whether it is upper or lower case

Genetic Terms• Dominant- 1 copy of

allele results in expression of trait, always use a capital letter to show it is dominant, ex. A

• Recessive-requires 2 copies of same allele to get expression of trait, always use lower case letter to show it is recessive, ex. a

• If purple is dominant to white flowers, how would you write your alleles?

Genetic TermsHomozygous Dominant-2 same copies of

dominant allele, get expression of dominant trait, AA

Heterozygous Dominant-1 copy of dominant allele, 1 copy of recessive, still get dominant trait expression, Aa

Homozygous recessive-2 copies of recessive allele, get expression of recessive trait, aa

Genetic Terms• Genotype-genes of an organism• Phenotype-physical appearance of the

organism

True or False: The phenotype of an organism is dictated by the genotype of that organism

• True breed lineage vs. hybrid offspring • P, F1, F2 Generation

Genetic TermsMonohybrid cross-a cross that only predicts

the genotype and phenotype for one trait, ex. plant flower color

Dihybrid cross-a cross that predicts the genotype and phenotype of 2 traits, ex. plant flower color and height

Predicting Gametes If an organism has the genotype BB, how

many different gametes can it make? 1: BWhen predicting gametes, keep in

mind that each gamete must have 1 complete set of instructions

Cc DDEE GgHh2: C, c 1: DE 4: GH, Gh, gH, gh

KKLlMM TTUUVV AaBBCc2 1 4

Mendel’s Early Genetic Experiments: Monohybrid Cross

Crossed a true breed purple flower plant with a true breed white flower plant

P generation PP x pp P-purple p-white

PpF1 generation, all purple heterozygous

dominantCrossed the F1 generation with each other

Pp x Pp

F2 generation PP, Pp, Pp, pp

Genotypic Ratio 1 PP: 2 Pp :1 pp

Phenotypic Ratio 3 purple: 1 white

Mendel’s Law of SegregationMendel continued his monohybrid

crosses (1000s) for 7 different traits

Average ratio for all traits studied was always about 3:1

The Law of Segregation: Each individual has 2 of every

gene (alleles) for each trait These genes (alleles) will

separate from each other during meiosis into different gametes.

Fertilization gives each new individual 2 alleles for each trait

A

b B

a

A

BA

b a

Ba

b

Genetics ProblemIn parrots, alleles for blue feathers are

dominant to alleles for yellow.

Cross a heterozygous dominant blue feathered parrot with a yellow feathered parrot

BbBb bb bb

BbBb bbbb

Genotypic ratio 1 Bb : 1 bbPhenotypic ratio 1 Blue : 1 yellow

b

b

B b

Genetics ProblemRhinoceroses can be born without a horn, the

recessive condition.

Cross 2 heterozygous dominant rhinoceroses. Can they produce a baby rhino without a horn?

HHHH HhHh

HhHh hhhh

Genotypic Ratio 1 HH: 2 Hh : 1 hhPhenotypic Ratio 3 with horns : 1 no horn

H-horn presenth-no horn

H h

H

h

Mendel’s Early Genetic Experiments: Dihybrid Cross

• Mendel noticed that all purple flower plants were tall and all white flower plants were short

• Could you ever see a purple short plant or a white tall plant?

• Crossed a homozygous dominant tall purple flower plant with a homozygous recessive short white flower plant

P generation AABB x aabb

F1 AaBb

F2 AaBb x AaBb

4 by 4 punnett square

Phenotypic ratio 9 purple, tall: 3 purple, short: 3 white, tall: 1 white, short

Mendel’s Early Genetic Experiments: Principle of Independent Assortment

• The results from the many dihybrid crosses allowed him to develop the

• Principle of Independent Assortment-gene pairs (traits) are independent of each other and are sorted into different gametes

• Exception: Linked genes; they always sort together into same gamete

A

b B

a

A

BA

b a

Ba

b

Diploid (2N)

Haploid (1N)

In panthers, black fur is dominant to yellow fur. A recessive gene results in the absence of claws.Predict the offspring of a cross between a heterozygous black panther with no claws and a yellow panther that is heterozygous for claws

BbCcBbCc

BbccBbcc

bbCcbbCc

bbccbbcc

Genotypic Ratio 1:1:1:1Phenotypic Ratio 1:1:1:1

bC bc

Bc

bc

B-black fur b-yellow furC-claws c-no claws

Punnett Square and Probabilities• What is the

probability from the following cross that any offspring will have unattached earlobes or attached earlobes?

• The probability of inheriting a specific allele is like flipping a coin and occurs every time parents have an offspring

Mendel’s Early Genetic Experiments: Test Cross

If you have a purple flower, what are the possible genotypes?

PP or Pp

How do you decide? Do a Test Cross using a white flower plant

PP x pp = all offspring are Pp (purple)

Pp x pp = ½ offspring are Pp (purple) ½ offspring are pp (white)

Human GeneticsThe study of inheritance and prediction of

genes in humansVery difficult, many genes involvedSmall sample size, few offspringMate by chance, live in diverse

environmentsLong life span makes it difficult to track

genes in different generations

Human Genetics: Understanding a PedigreePedigree-a chart of genetic connections between

individuals; family tree that tracks genes/diseasesSingle genes can be followed by constructing pedigreesSquare=male circle=femaleShaded =affected non-shaded=not

affectedA line between a circle + square=matingLines from the mating=offspring

Human Inheritances Patterns• Autosomal Recessive

Inheritance-gene is located on autosome; 2 copies of gene required for expression of disease/trait; 1 copy=carrier, not affected

• Albinism, cystic fibrosis, sickle cell anemia, phenylketonuria, methemoglobinemia, Niemann-Pick disease

Effects of Autosomal Recessive Disorders

FFFF FfFf

Ff Ff ffff

25% (1 out of 4) will have cystic fibrosisWhat are the genotypic/phenotypic ratios?

What are the chances of offspring from 2 heterozygote parents for the cystic fibrosis gene having the disease?

F-normal, no cystic fibrosis f-cystic fibrosis

Parents are carriers

F f

F f

Human Inheritances Patterns• Autosomal

dominant inheritance- gene is located on autosome; 1 copy of gene results in expression of disease/trait

• Ex. Achondroplasia, Huntington’s, Osteogenesis Imperfecta, polydactyly, progeria, sperocytosis

PPPP Pp Pp

PpPp pppp

75% (3 out of 4) chance of having polydactylyWhat is the genotypic/phenotypic ratios?

What are the chances of offspring from a cross of 2 heterozygous parents for polydactyly also having the condition?

P-polydactyly (extra fingers/toes) p-normal

P p

P

p

Variations to Mendelian Inheritance Patterns: Multiple AllelesCodominance2 alleles are not dominant to each other, and

if both are present both are expressedEx. Blood Groups- ABO blood typingA dominant to O, but not to BB dominant to O, but not to AO recessive

Codominance: Blood TypingThere are 6 genotypes that express 4

different blood phenotypes

Phenotype GenotypeType A AA, AOType B BB, BOType AB ABType O OO

Codominance: Blood TypingWhat does a blood typing do? Why is blood

called A, B, AB, or O?Based on the different sugars found on red

blood cells ex. Type A blood has A sugars on RBCs

What type of sugars does AB or O have?Why do we need to type blood?

Codominance: Blood Typing ProblemsCross a person with type O blood with one

that has type AB blood

Give phenotypic and genotypic ratios

AOAO AOAO

BOBO BOBO

Genotypic Ratio 1 AO : 1 BOPhenotypic Ratio 1 Type A : 1 Type B

O O

A

B

Type B blood male and a Type O blood female never produce a Type O blood child.

Is this possible? Why or why not?

BOBO BOBO

BOBO BOBO

Type B blood can be BB or BO*

Genotypic Ratio All are BOPhenotypic Ratio All are Type B*this genotype would produce a Type O offspring

O O

B

B

BOBO

OOOO

BOBO

OOOO

Genotypic ratio: 1 BO: 1 OOPhenotypic ratio: 1 type B: 1 type O blood

If father’s genotype is BO, it would be possible to get an O blood type child

B O

O

O

Variations to Mendelian Inheritance Patterns

• Incomplete Dominance-one allele isn’t complete dominant to the other; heterozygotes are intermediate in phenotype

• Snap dragon flower color: R-red, r-white

• RR x rr = Rr all offspring are pink

• What happens when 2 pink flower plants are crossed? Give phenotypic and genotypic ratios

Rr Rr

RRRR RrRr

RrRr rrrr

R rR

r

R-red r-white Rr-pink

Pleiotropy• Expression of alleles (for 1 trait) has positive

or negative effects on other traits Ex. Sickle cell mutation

• Affects the protein hemoglobin which carries O2

• Insufficient O2 will cause RBCs to sickle and eventually burst anemia

• Secondary effectsheart/lung damage, kidney/heart failure, skull deformation, mental impairment

Pleiotropy: Marfan SyndromeSingle gene mutation affects 2 or more distinct and

unrelated traits mutation of fibrillin gene

Incomplete PenetrancePolydactyly extra fingers/toesAutosomal dominant disorder which exhibits

incomplete penetranceA dominant allele sometimes does not

determine the phenotypeSome who inherit polydactyly allele are

phenotypically normal

Pleiotropy: Sickle Cell Anemia• Homozygous for condition die in

early 40s, no cure, extremely debilitating

• Severe anemia, poor circulation, physical weakness, impaired mental function, spleen damage

• Why is this mutation maintained?• Protection/resistance against malariaS-no sickle cell s-sickle cell

(recessive)• SS-no sickle cell, no resistance• Ss-no sickle cell, resistance• ss-sickle cell, resistance

Heterozygote advantage

Cross 2 heterozygous dominant parents together.

How many children would have sickle cell?

How many children would not?

How many children are resistant to malaria? How many are not?

SSSS SsSs

SsSs ssss

3 children do not have sickle cell, 1 does3 children protected, 1 is not

Parents are Ss-do not have sickle cell

S s

S

s

Epistasis• When one gene pair masks/prevents another

gene pair’s expressionEx. Labrador fur color• B-black fur b-brown fur• E-melanin deposited e-no melanin

• The recessive genotype of “ee” will cause no melanin deposition, thus the resulting fur coat will be yellow, even when “B or b” alleles are present

Continuous Variation in Traits

• Multiple genes are responsible for the phenotype of an organism polygenic inheritance

• Skin and eye color, height• A great deal of variation

exists resembling a bell shaped curve

• Look at human height; a few genes regulate height, but there exists a normal amount of variation

• What factors contribute to height?

Environmental Effects on Phenotype: Multifactorial Traits

Fur color on Siamese cats or Himalayan rabbits-heat sensitive enzyme that produces melanin

Flower color on Hydrangea Plant-influenced by acidity of soil

Height of Yarrow plant cuttings-varies depending on elevation planted

Continuous Variation in TraitsGenetics allows us to predict genotypes of

organisms, but there are many external and internal factors that influence the actual phenotype of those organisms cleft lip/palate, clubfoot, hypertension, diabetes, schizophrenia, allergies, cancers

An individual’s phenotype is the outcome of the complex interaction among all its genes and environment in which it lives

G x E interaction

Sex Determination in HumansDetermined by the 23rd pair on chromosomesXX-female XY-maleA female only makes eggs that carry the X

chromosomeA male makes sperm that contain either the X

or Y chromosomeMales are haploid for the X chromosome,

females are diploid

Sex Determination in Humans• Sex is determined by the Y chromosome• Up until about 7 weeks, an embryo has a

uncommitted reproductive duct system• This duct system will develop into

testes/penis if a Y chromosome is present• This is due to the expression of the SRY gene

located on the Y chromosome• No Y chromosome duct system forms into

ovaries/uterus• Faulty SRY phenotypic female, sterile

Human Inheritance Patterns

• X-linked recessive-gene is located on the X chromosome; 2 copies of gene required for expression in females, 1 copy in males results in expression. XX vs XY

• Ex. Fragile X syndrome, hemophilia, color blindness, muscular dystrophy, Menkes syndrome, adrenoleukodystropy

X-linked Recessive Inheritance Problem

What type of offspring would a colorblind man and woman who is a carrier for CB have?

XXCCXXcc XXCCYY

XXccXXcc XXccYY

Female offspring ratio 1 CB: 1 no CBMale offspring ratio 1 CB: 1 no CB

Female-Cc not colorblind Colorblind male-c But CB is X-linked XCXc XcY

Xc Y

XC

Xc

X-linked Recessive Inheritance Problem

Can a female with muscular dystrophy ever have a son who does not have MS?

Can a male with hemophilia have a daughter who is not affected with the disease?

XXMMXXmm XXmmYY

XXmmXXMM XXmmYY

No, all male offspring would have MS

M-no MS m-has MS

XM Y

Xm

Xm

XXHHXXhh XXHHYY

XXHHXXhh XXhhYY

Yes, female offspring would not have hemophilia, but are carriers for the gene

H-no Hemophilia h-Hemophilia

Xh Y

XH

XH