Patterns of Inheritance

The science of genetics has ancient roots

(400 BCE) An early explanation for inheritance suggested that particles called pangenes came from all parts of the organism and were incorporated into eggs or sperm to be passed to offspring.

(1800s)The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was suggested but later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.

Experimental genetics began in an abbey garden (mid 1800s)

Heredity is the transmission of traits from one generation to the next.

Genetics is the scientific study of heredity.

Gregor Mendel (1860s)

– Father of modern genetics

– Experimented with garden peas

– Found patterns to inheritance of traits

– Inheritance could be predicted

Experimental genetics began in an abbey garden

Mendel used pea plants. Why are they a good organism for genetic studies?

Mendel found evidence to support that– parents pass on to their offspring discrete “heritable factors” and – the heritable factors (today called genes), retain their individuality generation after generation.

Removal ofstamens

Carpel

White

Stamens

Transferof pollenPurple

Carpel maturesinto pea pod

Seeds frompod planted

Offspring(F1)

Parents(P)

2

3

1

4

Experimental genetics began in an abbey garden

True-breeding varieties result when self-fertilization produces offspring all identical to the parent.

The offspring of two different varieties are hybrids.

The cross-fertilization is a hybridization, or genetic cross.

Parental plants are the P generation.

Offspring/Filial = F1 generation.

A cross of F1 plants produces an F2 generation.

Genetics Vocabulary

Gene-segment of DNA that had the coding for a particular trait.

Allele- one of several varieties of a gene.

Locus- location on a chromosome where the gene is located

Testcross-mating between an individual of unknown genotype and a known(homozygous recessive individual)

… dominant, recessive, homozygous, heterozygous, phenotype, genotype

The Experiment

P generation(true-breedingparents)

F1 generation

F2 generation

of plantshave purple flowers

of plantshave white flowers

Purpleflowers

Whiteflowers

All plants havepurple flowers

Fertilizationamong F1 plants(F1 F1)

34

14

Character TraitsDominant Recessive

Flower color

Purple White

Flower position

Axial Terminal

Seed colorYellow Green

Seed shapeRound Wrinkled

Pod shape

Inflated Constricted

Pod color

Green Yellow

Stem length

Tall Dwarf

The seven pea characteristicsstudied by Mendel

Mendel’s Discoveries

• Principle of Dominance

• Law (Principle) of Segregation

• Law (Principle) of Independent Assortment

Mendel’s Theory of Segregation

An individual inherits a unit of information (allele) about a trait from each parent

During gamete formation, the alleles segregate from each other (homologous chromosomes separate from each other, one allele of each gene from each parent is passed to offspring)

Alleles

• Different molecular forms of a gene

• Arise by mutation

Homologous chromosomes bear the alleles for each character

A locus (plural, loci) is the specific location of a gene along a chromosome.

For a pair of homologous chromosomes, alleles of a gene reside at the same locus.

The law of independent assortment is revealed by tracking two characters at once

A dihybrid cross is a mating of parental varieties that differ in two characters.

Mendel performed the following dihybrid cross with the following results:– P generation: round yellow seeds wrinkled green seeds

– F1 generation: all plants with round yellow seeds

– F2 generation:

– 9/16 had round yellow seeds

– 3/16 had wrinkled yellow seeds

– 3/16 had round green seeds

– 1/16 had wrinkled green seeds

41

Sperm

Eggs

Yellowround

Greenround

Yellowwrinkled

Greenwrinkled

The hypothesis of independent assortmentActual results; hypothesis supported

RY

RY

rY

rY

Ry

Ry ry

ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYy rrYy Rryy rryy

41

41

41

41

41

41

41

169

163

161

F1 generation RrYy

163

F1 generation

41

41

41

41

All yellow round seeds(RrYy)

Meta-phase I

of meiosis

Anaphase I

Metaphase II

Fertilization

Gametes

F2 generation 9 :3 :3 :1

RY ry rY Ry

R

R

R

yy

y

rr

r

YY

Y

YY

Y

R

R R

r

rr

y

yy

Y

Y

y

y

R

R

r

r r

r

r

R

R

R

Y

Y

Y

y

y

y

Allelic combinationspossible in gametes

1/4 AB 1/4 ab 1/4 Ab 1/4 aB

Mendel concluded that the two “units” for a trait were to be assorted into gametes independently of any other “units” for the other traits

Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

Mendel’s Law of Independent Assortment

Genetics is more complicated than Mendel’s experiments lead him to understand….

Dominance Relationships: – Complete dominance (Simple dominance)

– Incomplete dominance

– Codominance

Multiple AllelesX-linked

Gene Interactions: - Pleiotrophy - Polygenic

Inheritance Patterns

Incomplete Dominance

XHomozygous parent

Homozygous parent

All F1 are heterozygous

X

F2 shows three phenotypes in 1:2:1 ratio

Incomplete Dominance- Example snapdragons

Another example of incomplete dominance in humans is hypercholesterolemia, dangerously high levels of cholesterol occur in the blood - heterozygotes have intermediately high cholesterol levels.

Blood Group(Phenotype) Genotypes

Carbohydrates Presenton Red Blood Cells

AntibodiesPresentin Blood

A

B

AB

O

IAIA

orIAi

IBIB

orIBi

IAIB

ii

Carbohydrate A

Carbohydrate B

Carbohydrate A

and

Carbohydrate B

Neither

Anti-B

Anti-A

Anti-B

Anti-A

None

No reaction Clumping reaction

O A B AB

Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left

Many genes have more than two alleles in the population (multiple alleles)

Human ABO blood group phenotypes involve three alleles for a single gene.

Chromosomes determine sex in many species

Many animals have a pair of sex chromosomes,• designated X and Y,

• that determine an individual’s sex.

In mammals,• males have XY sex chromosomes,

• females have XX sex chromosomes,

• the Y chromosome has the SRY gene for the development of testes, and

• an absence of the Y allows ovaries to develop.

Chromosomes determine sex in many species

Some organisms lack sex chromosomes altogether.

In bees, sex is determined by chromosome number.

• Females are diploid.

• Males are haploid.

In some species sex is determined by the temperature at which the eggs are incubated. (some crocodiles and turtles)

Sex-linked genes are located on either of the sex chromosomes.

The X chromosome carries many genes unrelated to sex. These genes are called X-linked.

Ex- The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.

Sex-linked genes exhibit a unique pattern of inheritance

Sex-linked traitsExamples- red-green colorblindness, Hemophilia, Duchenne’s muscular dystrophy

Cross a colorblind male with a female that is a carrier for the trait.

_______x _______

X- Inactiviation

Nondisjunction

A single character may be influenced by many genes

Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.

A single gene may affect many phenotypic characters

Pleiotropy occurs when one gene influences many characteristics.

Sickle-cell disease is a human example of pleiotropy. This disease• affects the type of hemoglobin produced and the shape of red

blood cells and

• causes anemia and organ damage.

• Sickle-cell and nonsickle alleles are codominant.

• Carriers of sickle-cell disease are resistant to malaria.

Non-Nuclear Inheritance

The environment affects many characters

Many characters result from a combination of heredity and the environment. For example,• skin color is affected by exposure to sunlight,

• susceptibility to diseases, such as cancer, has hereditary and environmental components, and

• identical twins show some differences.

Only genetic influences are inherited. Nature v. Nuture ( Genes v. Environment)

Environmental Effects on Plant Phenotype

• Hydrangea macrophylla• Action of gene responsible for floral color is

influenced by soil acidity• Flower color ranges from pink to blue

Environmental Effects on Plant Phenotype (Yarrow, Achillea millefolium)

Temperature Effects on Phenotype

• Rabbit is homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway

• Melanin is produced in cooler areas of body

Genetic traits in humans can be tracked through family pedigrees

In a simple dominant-recessive inheritance ; one allele is dominant and the other is recessive

P= purple p= white

Cross a heterozygous purple flowering plant with a plant that had white flowers.

Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.

Dominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobe

Examples of single-gene inherited traits in humans

Genetic traits in humans can be tracked through family pedigrees

A pedigree

Many inherited disorders in humans are controlled by a single gene

The most common genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is

– Recessive

– Carried by about 1 in 31 Americans.

– Shortened life expectancy (40-50 years)

Dominant human disorders include

– Achondroplasia, resulting in dwarfism, and

– Huntington’s disease, a degenerative disorder of the nervous system.

New technologies offer ways to obtain genetic information

– before conception,

– during pregnancy, and

– after birth.

Genetic testing can identify potential parents who are heterozygous carriers for certain diseases.

New technologies can provide insight into one’s genetic legacy

Several technologies can be used for detecting genetic conditions in a fetus.

– Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits

– karyotyping and

– biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease.

– Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests.

New technologies can provide insight into THE OFFSPRING’s genetic legacy

Amniocentesis

Ultrasoundtransducer

Fetus

Placenta

UterusCervix

Amniotic fluidextracted

Centrifugation

Amniotic fluid

Fetal cells

Culturedcells

Several hours

Several weeks

Several weeks

Biochemicaland geneticstests

Several hours

Several hours

Fetal cells

Cervix

Uterus

Chorionicvilli

PlacentaFetus

Ultrasoundtransducer

Tissue extractedfrom the chorionic villi

Chorionic Villus Sampling (CVS)

Karyotyping

Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects.

Fetal imaging, the most common procedure is ultrasound imaging, uses sound waves to produce a picture of the fetus.

Newborn screening can detect diseases that can be prevented by special care and precautions.

New technologies can provide insight into THE OFFSPRING’s genetic legacy

New technologies raise ethical considerations that include

– the confidentiality and potential use of results of genetic testing,

– time and financial costs, and

– determining what, if anything, should be done as a result of the testing.

New technologies can provide insight into one’s genetic legacy

Genes on the same chromosome tend to be inherited together

Linked genes, which

– are located close together on the same chromosome and

– tend to be inherited together.

Crossing over produces new combinations of alleles

Crossing over between homologous chromosomes produces new combinations of alleles in gametes / new chromosomes!.

Geneticists use crossover data to map genes

When examining recombinant frequency (crossing over), the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur.

Recombination frequencies can thus be used to map the relative position of genes on chromosomes.

•

Female MaleHemophilia

Carrier

NormalAlexis

Alexandra CzarNicholas IIof Russia

QueenVictoria

Alice Louis

Albert

Most compounds are synthesized by a sequence of metabolic steps involving many enzymes. If the enzymes (proteins) are not present or altered in some way (as in- the instructions coded for in the gene are altered) the compound will be affected.

In addition to simple gene inheritance; environmental factors, interactions between genes, mutations, even nutrition can have an effect on how a gene is expressed. This makes genetics a difficult and complex field of study.

1. Define and distinguish between these terms: the P generation, the F1 generation, and the F2 generation.

2. Define and distinguish between the following pairs of terms: homozygous and heterozygous; dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross and a Punnett square.

You should now be able to

3. Name and explain Mendel’s laws.

4. Describe the structure of homologous chromosomes.

5. Explain how family pedigrees can help determine the inheritance of many human traits.

6. Explain how recessive and dominant disorders are inherited. (be able to use a Punnett square)

7. Describe the types and use of fetal testing: amniocentesis, chorionic villus sampling, and ultrasound imaging.

You should now be able to

8. Describe the inheritance patterns of incomplete dominance, multiple alleles, codominance, pleiotropy, and polygenic inheritance. Be able to do crossed involving incomplete dominance and ABO blood group.

You should now be able to

10. Define the term: linked genes.

11. Explain how sex is genetically determined in humans and the significance of the SRY gene.

12. Describe patterns of sex-linked inheritance and examples of sex-linked disorders. Be able to do a Punnett Square.

You should now be able to