Genetics Since Mendel

1
Genetics Since Mendel

• Chapter 5, Section 2

2
Genetics Since Mendel

• What have we found out since Mendel?

• Since Mendels experiments, scientists have found
  that genetic inheritance can be much more
  complicated than the simple, dominant-recessive
  inheritance that Mendel saw in pea plants.

3
Incomplete Dominance

• Incomplete Dominance

• Occurs when offspring of two homozygous parents
  show an intermediate phenotype.
• The heterozygous form is in between the dominant
  and recessive homozygous forms.

4
Incomplete Dominance

• Incomplete Dominance

• Example
• Snapdragons
• Crossing a purebred white snapdragon with a
  purebred red snapdragon produces a pink
  snapdragon.

5
Multiple Alleles

• Multiple Alleles

• Trait is controlled by more than two alleles.
• Produce more than 3 phenotypes for the trait.

6
Multiple Alleles

• Multiple Alleles

• Example
• Blood Type
• Alleles IA IB and i
• Phenotypes A, B, AB, O
• Type O is recessive to types A and B.
• Types A and B are both expressed in the phenotype
  when paired together.

7
Polygenic Inheritance

• Polygenic Inheritance

• Occurs when a group of gene pairs act together to
  produce a trait.
• Traits are controlled by alleles from more than
  one gene.
• A wide variety of phenotypes are produced.
• Examples
• Hair, eye, and skin color

8
Impact of the Environment

• Environmental influence on genetics

• Environmental influences can be internal or
  external.
• Examples
• A boy who has genetics to be six feet tall is
  malnourished as a child and is only five foot
  eight inches tall.
• Male birds exhibit brighter feathers than female
  birds.

9
Human Genes and Mutations

• What is a mutation again?

• Error in the DNA sequence.
• Can be harmful, helpful, or null.
• Can be caused by mistakes in replication or by
  external agents called mutagens.

10
Human Genes and Mutations

• Chromosome Disorders

• An organism receives too many or too few
  chromosomes as a result of mistakes in meiosis.
• Usually fatal
• Example
• Downs syndrome
• 3 copies of chromosome 21.

11
Recessive Genetic Disorders

• Recessive Genetic Disorder

• Disorder only appears if an individual has both
  recessive alleles.
• Parents are carriers (heterozygous) and do not
  show symptoms of the disorder.

12
Recessive Genetic Disorders

• Recessive Genetic Disorder

• Example
• Cystic fibrosis
• Production of thick instead of thin mucous in the
  lungs and intestinal tract.
• Most common genetic disorder leading to death in
  Caucasian Americans.

13
Determination of Gender

• Boy or Girl?

• Sperm cells have either an X or a Y chromosome.
• Egg cells have only an X chromosome.
• Males XY
• Females - XX

14
Sex-Linked Disorders

• Sex-linked Disorders

• Alleles controlling the genetic disorder are
  linked to either the X or the Y chromosome.

15
Sex-Linked Disorders

• Sex-linked Disorders

• Example
• Color blindness
• Recessive allele linked to the X-chromosome.
• All males with the recessive allele are
  colorblind.
• Females must have two recessive alleles to be
  colorblind.

16
Pedigrees

• Pedigrees

• Family tree for a certain trait.
• Used to determine inheritance patterns and
  probabilities and in breeding plants and animals.

17
Pedigrees

• How to read a pedigree

• Males squares
• Females circles
• Have the trait colored
• Does not have the trait empty
• Is a carrier (heterozygous) half-colored