Patterns of Gene Inheritance. Genetics is the study of genes What is a gene? A. A factor that controls a heritable characteristic B. Something on a chromosome.

Patterns of Gene Inheritance

Genetics is the study of genes

What is a gene?

A. A factor that controls a heritable characteristic

B. Something on a chromosome

C. Information stored in a segment of DNA

D. Something that encodes a protein

Gregor Mendel

LOTS O’ DEFINITIONS

• Gene---A piece of DNA that encodes a particular trait. EX a gene for eye-color

• Allele—an alternate form of a gene. EX—allele for blue eyes and allele for brown eyes

• Phenotype—the physical expression of a gene or allele. Ex. Blue eyes

• Genotype—the genetic composition of an individual.

MORE DEFINITIONS• Locus—the location of a gene on a

chromosome. Plural=loci

• Dominant allele—expressed whether alone or in pairs. Symbolized by a capital letter. EX Brown eye allele=B

• Recessive allele—expressed only in the absence of a dominant allele. Symbolized by lowercase. EX blue eye allele=b

• What is the maximum # of alleles a diploid individual can have at any given locus?

STILL MORE DEFINITIONS!!!

• Genotype—the genetic composition of an individual– Homozygous----containing a pair of the same

alleles. Can be

• Homozygous recessive—two recessive alleles EX. bb, or

• Homozygous dominant– two dominant alleles EX BB

– Heterozygous—containing two different alleles. EX Bb

Law of Segregation

• Each individual has two factors (called genes today) for each trait.

• Factors segregate during gametogenesis. Ploidy level of gametes?

• Fertilization gives each new individual two factors again.

• Ploidy level after fertilization?

Gene locus

Defs

Example--Widow’s Peak

Gametogenesis

• Homologous pairs separate during meiosis. When? a gamete has only one allele from each pair

of alleles. If the primary spermatocyte is genotype Ww,

the sperm cell would contain either a W or a w, but not both.

• Ww represents the genotype of an individual.

• Gametes are represented by W or w.

One-Trait Crosses

GENOTYPE?GENOTYPE?

GENOTYPE?

Punnett SquareEXAMPLE 1—A Monohybrid Cross

Monohybrid cross

The One-Trait Testcross

QUESTION: How can we determine the QUESTION: How can we determine the genotype of a dominant phenotype?genotype of a dominant phenotype?

Testcross: cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).

The Law of Independent Assortment

• Law of Segregation involves 1 character. What about 2 (or more) characters?

• Each pair of alleles segregates independently of the other pairs

• All possible combinations of alleles can occur in the gametes

• EX: two pairs of alleles segregate independently of each other.

• When will different alleles not sort independently?

E

n

e

N

Father

Mother

N

eE

n

e

n

E

N

e

n

E

N

e

N

E

n

Telophase II

Replication

Independent Assortment

N

E

n

e e

n

E

N

OR

Alignment of Homologs at Metaphase I

Segregation and Independent Assortment

MEIOSIS & GENETIC VARIATION

Independent Assortment

• Diploid organisms can produce 2n diff. gametes

• Ea. homologous pair can orient in two different ways

• Humans: 223 = 8,388,608

(est. 8.4 million)

Each homologous pair can orient in either of two ways

HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT

Gene for

browneyes

Eye color

Gene for

blueeyes

Gene for

black hair

r

Gene for red hair

Hair color

INDEPENDENT ASSORTMENT

OR

During meiosis I, tetrads can line up 2n different ways.

Brown eyesBlack hair

Blue eyesRed hair

Brown eyesRed hair

Blue eyesBlack hair

Meiosis I & II

E

n

e

N

Father

Mother

N

eE

n

e

N

E

n

Replication

Linked Genes and Independent Assortment

N

E

n

E e

n

e

N

OR

Alignment of Homologs at Metaphase I

L l

l

lLL

L L ll

What are the possible genotypes of this individuals gametes?

Dihybrid cross

Probabilities

Two-trait Testcross-How?

Genetic Disorders• Pedigree charts- show patterns of inheritance

Autosomal Dominant Disorders

• Autosomes---Non-sex chromosomes

• Dominant allelewhen under what genotype(s)will an

individual show the disorder?

Examples: Autosomal Dominant Disorder

• Neurofibromatosis– Small benign tumors, – Gene on chromosome 17.

• Huntington Disease– Progressive degeneration of the nervous

system– Normal until middle age– No treatment– Gene has been isolated (chromosome 4),

genetic testing can diagnose

Huntington disease

Autosomal recessive pedigree chart

• Autosomes---Non-sex chromosomes

• Recessive allelewhen under what genotype(s)will an

individual show the disorder?

Autosomal Recessive Disorders

• Tay-Sachs Disease– Common among United States Jews of central and

eastern European descent. – Death by the age of three or four. – Defective enzyme in lysosomes.

• Cystic Fibrosis– Most common lethal genetic disorder among

Caucasians– Defective chloride ion transport protein – Osmotic imbalance results in thick mucous in lungs

and pancreatic ducts

Cystic fibrosis therapy

Phenylketonuria (PKU)• allele on chromosome 12.

• lack an enzyme needed for metabolism of phenylalanine (an amino acid)

• Urine test diagnostic.

• Brain damage unless controlled by diet.

• Polygenic (Quantitative) Traits

• Governed by more than one gene pair.

• Several genes determine the phenotype.

• Produce bell-shaped curve.

• EX: Skin color

Polygenic Inheritance

EX 2 Polygenic Inheritance :

Polygenic Disorders

• Examples: cancer, schizophrenia, hypertension, diabetes, etc– several genes involved– also environmental influences.

Multiple Allelic Traits

• Often more than two alleles exist for a particular gene locus.

• Maximum # of alleles at any given locus?

• Each individual inherits only two alleles for these genes!!!

• EX: Human Blood type

Inheritance of blood type

Details

Incompletely Dominant Traits• Codominance--both alleles equally

expressed in a heterozygote. EX ABO Blood type

• Incomplete dominance--heterozygote shows intermediate phenotype, representing a blending of traits. EX Wavy hair in Caucasians

• Phenotypic ratio ?• 1 : 2 : 1.

Incomplete dominance

Example 2-Incomplete Dominance

Sickle-Cell Disease• Caused by incompletely dominant alleles.• Single nucleotide mutation causes abnormal

hemoglobin.• HbA = normal hemoglobin• HbS = sickled condition.• Genotype of normal?• Genotype of sickled homozygote?• Genotype of intermediate phenotype?

Sickle-Cell Disease

Molecular Explanation-Dominance

• Simple Dominance– One-half the amount of gene product sufficient for

phenotype

• Incomplete Dominance– Recessive allele not expressed in heterozygote– Homozygous dominant –2 doses, full phenotype– Heterozygote—1 dose, reduced phenotype

• Codominant– Both alleles expressed, combined phenotype

Let’s Get Small!!!

Everything’s Relative

WHAT’S HAPPENING??

SUMMARY• Physical Basis of Genes• Lots o’ Definitions• Law of Segregation• Law of Independent Assortment• One-Trait Crosses• Law of Independent Assortment• Two-Trait Crosses• Genetic disorders• Polygenic Traits• Incomplete Dominance/Codominance

REMEMBER!!!• Relate genetics to meiosis

• For genetics problems—first try to figure the genotype(s) of the gametes