single gene inheritance -- mendelian

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

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

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

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.

Gene locus

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.

One-Trait Crosses

GENOTYPE?

GENOTYPE?

Punnett Square

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?

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.

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)

INDEPENDENT

ASSORTMENT

OR

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

Brown eyes Black hair

Blue eyes

Red hair Brown eyes_{Red hair}

Blue eyes Black hair

E n e N Father Mothe r N e E 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 l L L

What are the possible genotypes of this

individuals gametes?

Dihybrid

cross

Genetic Disorders

Autosomal Dominant Disorders

• Autosomes---Non-sex chromosomes • Dominant allele

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

Autosomal recessive pedigree

chart

• Autosomes---Non-sex chromosomes • Recessive allele

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

Phenylketonuria (PKU)

• allele on chromosome 12.

• lack an enzyme needed for metabolism

of phenylalanine (an amino acid)

• Urine test diagnostic.

• Polygenic (Quantitative) Traits

• Governed by more than one gene pair. • Several genes determine the phenotype. • Produce bell-shaped curve.

• EX: Skin color

Polygenic Disorders

• Examples: cancer, schizophrenia,

hypertension, diabetes, etc

– several genes involved

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!!!

Inheritance of blood type

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

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?

Molecular Explanation-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

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

REMEMBER!!!

• Relate genetics to meiosis

• For genetics problems—first try to

figure the genotype(s) of the