Applying Mendelian Genetics

Meiosis

Review by reading pages 84 – 87 and 265 - 271

SEX!

Male – XY therefore sperm is either X or YFemale – XX therefore egg is only XSigns of sex emerge when embryo is 2

months old. Prior to this gonads are “generic”SRY gene on Y chromosome causes gonads to

form testes and codes for a protein that regulates other genes which control testis function.

In females, one of the 2 X chromosomes becomes inactivated when a methyl group attaches to it. This inactivated X is a Barr body

Which X, in each embryonic cell becomes inactive, is random so each female is a mosaic of cells some with the active X from the father and others from the mother.

A few genes on the inactive chromosome remain active.

Sex Linked Traits

Trait found on X chromosomeMales are most often affected because they

have just one X chromosome.Females have 2 X’s so if trait is recessive a

healthy gene will mask expression of the disease causing gene

A female with one diseased X chromosome is a carrier

Examples) color blindness, hemophilia

Punnett Squares

Used to predict probability of producing a particular genotype/phenotype when parental genotype is known.

See pages 92-94 (IB book) for monohybridSee pages 272-273 (IB book) for dihybrid

Complete Dominance - Phenotypes of the heterozygote and the homozygous dominant are indistinguishable.

Codominance – the two alleles affect the phenotype in separate, distinguishable ways.

Ex) MN blood group in which the heterozygote has both the M and N molecules on the RBC’s

Dominance does not mean the trait is more common in the population. Ex.) polydactyly

The allele for polydactyly is dominant to the allele for 5 digits. Most people are recessive homozygotes for this trait.

Linked Genes10.2.4 – 10.2.6

Located on same chromosome – tend to be inherited together.

A linkage group is a group of genes inherited together because they are found on the same chromosome

Give results that deviate from expected Mendelian ratios.

Example – page 275 – 277 in IB Book

Multiple Alleles

Most genes have more than two forms of an allele.

Ex.) ABO blood types3 alleles: IA and IB are codominant and i is

recessive.

Pleiotropy

A single gene affects an organism in many ways.

Ex.) sickle cell disease

Polygenic InheritancePage 278-281

2 or more genes influence the expression of one trait

Examples are skin pigmentation or height.For skin, it is believed there are at least 3

separately inherited genes in which the dominant alleles A, B, and C each contribute one unit of darkness to the phenotype.

AaBbCc would be the same in pigmentation as AABbcc, each with 3 dark units.

Environment Impacts Expression of Phenotype

Nature vs. NurtureThe product of a genotype is a range of

phenotypic possibilities over which there may be variation due to environmental influence.

This phenotypic range is called norm of reaction for a genotype. Ex.) hydrangea flowers vary in color due to soil acidity.

Norms are broadest for polygenic traits.Multifactorial – many factors both genetic and

environmental influence many phenotypes

Human Inheritance

Scientists cannot manipulate mating in humans, therefore, they study families using pedigrees.

Pedigrees can be used to predict the outcome of future matings.

Pedigrees can often determine the genotype of family members for a particular trait.

PEDIGREE

Human Disorders

Pedigree Analysis4.3.12

http://www.hhmi.org/biointeractive/vlabs/cardiology/content/dtg/pedigree/pedigree.html

Page 98 in IB book

Recessive Disorders

These traits appear only in homozygous recessive individuals (aa).

AA and Aa are phenotypically normal.Aa are considered carriers as they can pass

the trait to their offspringAlbinism – lack of pigmentationCystic fibrosis – defective or absent chloride

channels result in abnormally high concentration of extracellular chloride which makes mucus thick and sticky.

Recessive disorders

Tay-Sachs disease – brain lipids build up due to a dysfunctional enzyme. Causes seizures, blindness, and degeneration of motor and mental abilities.

Sickle cell disease – caused by the substitution of a single amino acid in hemoglobin of the RBC. When oxygen levels are low hemoglobin crystallizes changing shape of RBC. Heterozygotes have sickle cell trait and both normal and abnormal hemoglobin are made.

Consanguinity increases the risk of inherited disorders because individuals are more likely to carry same recessive allele.

Some populations mate with/marry close relatives due to religious reasons or geographic isolation. These populations are often used by modern scientists to study genetic disorders.

Dominant Disorders

Achondroplasia – a form of dwarfism. 99.99% of the population is homozygous recessive for the allele.

Lethal dominant disorders are rare because those affected often die before reproduction.

Huntington’s disease – a late acting disease which does not affect an individual until they have had offspring. Causes nervous system deterioration which is irreversible and fatal.

Multifactorial Disorders

Have a genetic and environmental influence.Genetic component may be polygenic.Best strategy is to educate public about

environmental impact and behavioral strategies for avoiding these diseases.

Ex.) heart disease, high cholesterol, cancer, alcoholism, etc…

Genetic testing and counseling

Pedigree analysis and laws of probabilityGenetic screening of parentsFetal testing

Amniocentesis Chorionic villus sampling Ultrasound Fetoscopy

Newborn screening – detects disorders in newborns so disorder can be treated.