PATTERNS OF HEREDITYAND HUMAN GENETICS

CHAPTER 11

☺ You will compare the inheritance of recessive and dominant traits in humans.

☺ You will analyze the inheritance of incompletly dominant and codominant traits.☺ You will determine the inheritance of sex -

linked traits.

Mendelian Inheritance ofHuman Traits

Making a Pedigree▪Pedigrees illustrate inheritance: a graphic

representation of genetic inheritance.▸Made up of a set of symbols that identify males, females and the

trail being studied and family relationships.

☐= male; = ❍ female

┬ ☐ ❍ = parents ▄ affected male

┌┴┐

●affected female

☐ ❍ = siblings Half shaded square or

circle means a carrier of trait

Simple Recessive Heredity▪Most genetic disorders are caused by recessive

alleles. Allele must be inherited from both parents.

▪Many of the alleles are relatively rare.▪Few are common in certain ethnic groups▸Cystic fibrosis - most common genetic disorder among

white Americans. 1 in 20 carry, 1 in 2000 inherit disorder.▸Tay-Sachs - Amish and Ashkenzic Jews. Disorder of

the central nervous system.▸Phenylketnuria (PKU) - Norway, Sweden. Disorder

that results from the absence of an enzyme that converts an amino acid to another amino acid.

Simple Dominant Heredity

▪Trait only has to be inherited from one parent▸Simple dominant traits - Tongue rolling, hitchhiker’s

thumb, attached earlobes, and Hunington’s disease.

When Heredity FollowsDifferent Rules

Complex Patterns of Inheritance

▪Incomplete dominance: a third genotype: A dominant red flower and a dominant white flower produce a pink flower.▸Both flowers have incomplete dominance patterns, neither allele

is dominant.

▪Codominance: Expression of both alleles.▸The phenotype of both homozygotes to be produced in

heterozygote individuals. Both alleles are expressed equally.▪Traits controlled by more than two alleles have

multiple alleles.

Sex Determination

1:1 Ratio; 50 - 50 Chance ▪22 pairs of matching homologous chromosomes

called autosomes; look exactly alike.▸23rd pair differ in males and females. Females have a

“XX” pair males have a “XY” pair.–These are the chromosomes that control the inheritance of sex

characteristics.

▸Females only produce the “X” gamete while males produce both the “X” and the “Y” gamete.

Sex-linked Traits & Polygenic Inheritance

Traits carried on the 23rd pair

▪Traits dependent on genes that follow the inheritance pattern for a sex chromosome.▸Color blindness

▪Traits that are controlled by two or more genes▸May be on the same chromosome or on different

chromosomes–Skin color, height .

Environmental Influences

External and Internal

▪External influences: Temperature, nutrition, light, chemicals, infection all can change the way a gene is expressed.

▪Internal influences: hormones and structural differences, organisms age can change a gene’s expression.

Complex Inheritance