Chapter 12

Inheritance Patterns and Human Genetics

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I. Sex DeterminationA. Autosomes: are matching chromosomes, in humans there are 22 pairs.

B. The 23rd pair are the sex chromosomes.Females are XX Males are XY

II. Effects of Gene Location:A. Sex – linked: Traits that are carried on the X chromosome, not the Y. Colorblindness and Hemophilia are two examples.

–Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked trait.

The Red X represents one carrying a defect such as hemophilia

B. Linked genes– Pairs of genes that tend to be inherited together

are called linked genes. C. Chromosome Mapping– The farther apart two genes are located on a

chromosome, the more likely a cross-over will occur.

– Researchers use recombinant percentages to construct chromosome maps showing relative gene positions.

III. Mutations Germ-cell mutations occur in gametes and

can be passed on to offspring. Somatic-cell mutations occur in body

cells and affect only the individual organism. Chromosome Mutations

– Chromosome mutations are changes in the structure of a chromosome or the loss or gain of an entire chromosome.

Gene MutationsGene mutations are changes in one or more of the nucleotides in a gene.

IV. Inheritance of Traits Pedigrees

– Geneticists use pedigrees to trace diseases or traits through families.

– Pedigrees are diagrams that reveal inheritance patterns of genes.

Pedigree is a chart that shows the inheritance of a trait.

Question of the day: Color blindness is a sex-linked traitthat is carried on the X chromosome. If a

boy is born color-blind, what would have to be true?

A. His father had normal vision.B. His grandmother was colorblind.C. His mother carried at least onegene for color blindness.D. His grandfather passed on thecolor-blind trait to his father

V. Genetic Disorders Huntington’s Disease: Rare

dominant gene. Results in brain and nervous system deterioration. Onset between the ages of 30 – 50.

Cystic Fibrosis: Recessive gene. Build up of mucous in the lungs and digestive tract. Pneumonia, poor digestion. Onset at birth

Typical Pedigree of Huntington’s Disease

I

II

III

Tay-Sachs disease: Recessive gene, Lipid Accumulation in the brain, seizures, blindness,Onset at birth.PKU: Recessive gene, Inability to digest Phenalanine, mental retardation, Onset birth.Can be controlled with diet.

Sickle-Cell Anemia: Co-dominant genes. Deformed hemoglobin with the inability to properly carry oxygen. Onset birth. If one geneis normal and one sickle cell – ½ the blood will be normal. Has been treated with stemcells.

Color blindness: Sex-linked gene. Inability todistinguish hues.Hemophilia: Sex-linked. Missing factor 9 theprotein need for blood to clot.Down’s Syndrome: Nondisjunction of the 21st

chromosome. Trisomy 21. Developmentallydisabled, some medical problems at birth.

Klinefelter’s Syndrome: Nondisjunction of thesex chromosomes. XXY, males that are developmentally disabled.Turner’s Syndrome: Nondisjunction of the sex chromosomes. Single X. Female, failure to develop sexually without medication.

Cri-du-chat: Large section of chromosome 5missing. Extremely small head, rarely survivemore than 1 to 2 years.

Most chromosomal mutations are lethal

Karyotype is a picture of one’s chromosomes.

Amniocentesis can be done during pregnancy and the a karyotype made to see any Chromosomal abnormalities.

VI. When Heredity Follows Different Rules

A. Incomplete Dominance: The traits blend, neither trait is expressed fully.

B. Co dominance: Both alleles are expressed equally.Cross a black chicken with a white rooster and you

get a black and white chickenBlood type is also co dominant. AB have equal effect,

O is a recessive gene.

C. Multiple Alleles: When there are more than two possible alleles for a trait. Blood type again is an example, three possible alleles A, B or O.

D. Polygenic Traits: Takes more than one gene pair to code for a trait.

*Height requires 3 gene pairs

*Hair, eye color and skin tone require 7 gene pairs.

E. Sex-Influenced traits: Traits that are controlled by the hormones of the body.

Examples: lion’s mane, a man’s beard, woman’s breast development, horn size on animals, feather color in birds.

Baldness in males

F. Environmental Controls

Genes can fail in their expression if the environmental conditions are not present.

Temperature, nutrition, light, chemicals, infectious agents.

Negative genes can be expressed if conditions are also right. Genes that cause alcoholism.

You do not need to write this!!!

The sex of alligators is determined by nest temperature early in the incubation period (from 7th to 21st day). If the nest temperature is below 86 degrees, all hatchlings are female. Above 93 degrees, all hatchlings are males. At temperatures in between there are both males and females, usually with a greater percentage of females.

http://www.centralfloridazoo.org/animals/American_alligator.htm

Complex Inheritance of Human Traits

Genotype Surface Protein Phenotype

IAIA, IAi A A

IBIB, IB i B B

IAIB AB AB

ii NONE O

A. Blood Typing is an example of Co-dominance and multiple Alleles.

B. A, B are dominant to O

Rh factor: + is dominant, negative is recessive.Person that is positive could be ++ or +-Person that is negative would be –

AB = universal recipient, O = universal Donor

Surface molecule B

Surface molecule A

Gene Therapy– In gene therapy, a defective gene is

replaced with a copy of a healthy gene.– Somatic cell gene therapy alters only

body cells. – Germ cell gene therapy attempts to alter

eggs or sperm.