Chapter 14-Mendel and the Gene Idea

Chapter 14
Mendel and the Gene Idea

Father of Genetics
Gregor Mendel

Grew up in Austria and entered the monastery where he became a monk

Went to the University of Vienna where he studied math and botany

Began his genetic studies round 1857 using the common garden pea plant

Mendel studied many characters (genes)of the pea plant such as: Flower color (purple or white) seed color (yellow or green) seed shape (round or wrinkled) and height (tall or short)

Trait- a variant of a particular character such as: the character for flower color has the traits purple or white

Reproductive Parts of the Plant

Stamen- the pollen producing male reproductive organ of a flower, consisting of an anther and a filament.

Carpel- the female reproductive organ of a flower,consisting of the stigma, style and ovary.

Mendel’s First Cross

Purple flowered plants x White flowered plants = All purple offspring

What does this tell us about the genes for purple and white?

That the purple gene is ___________

And the white gene is ___________

Mendel’s Laws

1.Alternative versions of genes (alleles) account for variations in inherited characters

2.For each character, an organism inherits two alleles, one from each parent

3.An allele may be dominant or recessive

4.Homologous chromosomes and their genes segregate during meiosis

Dominant and Recessive Genes

REMEMBER:

For a dominant gene to express itself you only need one copy of the gene. Example: Pp

For a recessive gene to express itself you need two copies of the gene. Example: pp

Some Genetic Vocabulary

Homozygous

Heterozygous

Phenotype

Genotype

Test cross

A Dihybrid Cross Shows the Law of Independent Assortment

Independent Assortment- The genes for one trait (seed color) are distributed to the gametes independently of the genes for for the other trait (seed shape)

Yy

Rr

Y= yellow y= green

R= round r= wrinkled

What genes you inherit follow the laws of probability

The probability scale ranges from 0-1

1= a high chance of something happening

0= a low chance of something happening

0 .5 1 (some events in-between 0-1)

Examples- a two headed coin

a regular coin

Boy or Girl

Incomplete Dominance
(When both Genes are Dominant)

A type of inheritance in which the F1 hybrids have an appearance that is intermediate between the phenotypes of the parents. You get a blending effect.

Fig 14.9 – Snapdragons

RR x WW

R= gene for red flowers

W= gene for white flowers

Codominance

In this type of inheritance both genes are again dominant but instead of a blending type effect as in incomplete dominance both genes are completely expressed in the offspring.

Example Blood type AB is produced from a mating between a pure type A blood and a pure type B blood

AA x BB>>>>>>> AB

MM x NN>>>>>>>MN

Multiple Alleles

Multiple Alleles- genes that exist in more than 2 forms

Example- genes that control blood types exist in 3 forms:

A (Dominant)

B (Dominant)

O (Recessive)

Blood Types
A and B are dominant genes

Polygenic Inheritance

Polygenic Inheritance- it is the additive effect of 2 or more genes on a single phenotypic character.

Example- skin color is controlled by 3 pairs of genes, all of which are dominant.

AA aa Aa

BB bb Bb

CC cc Cc

Pleiotropy

Pleiotropy- the ability of one gene to affect an organism in many ways

Sickle Cell Anemia- defective hemoglobin protein. Valine has replaced glutamic acid in the amino acid sequence.

Sickle Cell Trait-1 in 10 African Americans is a carrier. Helps protect against malaria

Sickle Cell Anemia. 1 in 400

In Sickle Cell One Amino Acid Is Out of Place

Val-His-Leu-Thr-Pro-Glu-Glu Normal

Val-His-Leu-Thr-Pro-Val-Glu Sickle-Cell

Other Genetic Diseases

Cystic fibrosis- a recessive disease affecting 1 in 2500 whites. Mucus builds up in lungs,digestive tract. Infections set in.

Tay- Sachs- 1 in 3600 Ashkenazic Jews.Here the recessive defective gene causes lipids to build up in the brain

Albinism- here the recessive gene causes a lack of skin pigment. Skin and hair white.Eyes-no color.

Huntington’s Disease- this is caused by a dominant gene.Destroys the brain. Expressed late in life-(35-50yrs)

Multifactorial Disorders =
genes+environment

Even though you may have inherited a bad gene you can modify or change your environment to slow down the effects of that gene

Heart Disease

Cancer

Alcoholism

Diabetes

Pedigree Analysis

Pedigree Charts- a family tree describing the occurrence of heritable characters in parents and offspring across as many generations as possible.

Example –Two apparently normal people marry and have a child with sickle cell anemia (SS)

Carrier, Fetal and Newborn Detection

Carrier Detection

Fetal Testing- Amniocentesis, Chorionic Villus sampling, Fetal cells from mothers blood. Ultrasound, Fetoscopy

Newborn Screening- PKU. Phenylketonuria- here you have a genetic defect in which you cannot break down the amino acid phenylaline. 1in 10,000 births

DNA Fingerprints

Just as each person has a unique set of fingerprints on their hands, they also have a unique DNA sequence. (the A’s, T’s G’s and C’s that make up your DNA molecules)

Genetics Problem

What are the chances that a brown eyed man who is also carrying the gene for blue eyes will have a blued eyed child if he marries a woman who has the same genotype as he does?

Let B = dominant gene for Brown eyes

Let b = recessive gene for Blue eyes

What are the chances of them having a blue eyed girl?

A brown eyed boy?

A man with group A blood marries a woman with group B blood. Their child has group O blood.

What are the genotypes of the parents?

What other blood types can the children of these parents have?