Chapter 11

1
Chapter 11Mendel and Heredity

• Heredity is the passing of traits from parents to
  their offspring.
• The study of heredity is called Genetics.
• Gregor Mendel was an Austrian monk who first
  studied heredity by breeding different varieties
  of garden peas.
• Mendel is considered the Father of Genetics
  because he was the first to develop rules that
  accurately predict patterns of heredity.

2
Useful Features in Peas

• Seven traits that have two clearly different
  forms that are easy to tell apart.
• For example, the flower color is either purple or
  white- there are no intermediate forms.

3

• 2. Mating of the garden-pea flowers are easily
  controlled-
• Both male and female gametes are on the same
  flower.

4

• Mendel carefully cross-pollinated his pea plants
  by removing the stamen (male reproductive organ
  that produces pollen) from the flower of one
  plant.
• Then he dusted the pistil (female reproductive
  organ that produces eggs) of that plant with
  pollen from a different pea plant

5

• 3. The garden pea is small, grows easily, matures
  quickly and produces many offspring.

6
Mendels Work Became a Theory of Heredity

• Before Mendels experiments, many people thought
  offspring were just a blend of the
  characteristics of their parents.
• Mendels results did not support the blending
  hypothesis. He correctly concluded that each pea
  has two separate genes for each trait- one from
  each parent.

7
Mendels Hypotheses

• For each inherited trait, an individual has two
  copies of the gene- one from each parent.
• There are alternative versions of genes. For
  example, the gene for flower color can be purple
  or white. Different versions of a gene are called
  alleles.
• When two different alleles occur together, one of
  them may be completely expressed, while the other
  may have no effect on the organisms appearance.
• The expressed forms of a trait is dominant.
• The trait that is not expressed when a dominant
  trait is present is described as recessive.

8

• 4. When gametes are formed, the alleles for each
  gene in an individual separate independently of
  one another.
• So, gametes only carry one allele for each trait.

9
Genetics

• Dominant alleles are written by capitalizing the
  first letter of the trait.
• Recessive alleles are also written using the
  first letter of the trait but it is not
  capitalized.

10

• If the two alleles of a particular gene present
  in an individual are the same, the individual is
  homozygous for that trait.
• If the two alleles of a particular gene are
  different, the individual is heterozygous for
  that trait.
• In heterozygous individuals, only the dominant
  allele is expressed.

11

• The physical appearance of a trait is called a
  phenotype
• The set of alleles that a person has is called
  the genotype

12
Mendels Ideas Gave Rise to the Laws of Heredity

• Law of Segregation- during meiosis, the two
  alleles for a trait separate when gametes are
  formed.

13

• Law of Independent Assortment- the inheritance of
  one trait does not influence the inheritance of
  any other trait.
• For example, the alleles for plant height
  separate independently of the alleles for flower
  color.

14
Studying Heredity

• Punnett square is a diagram that predicts the
  expected outcome of a genetic cross by
  considering all the possible combinations of
  gametes in the cross.

15

• Monohybrid Cross- a cross that considers one pair
  of contrasting traits between two individuals.
• Use a punnett square to predict the outcome of a
  cross between a pea plant that is homozygous for
  yellow seed color (YY) and a pea plant that is
  homozygous for green seed color (yy).

16
Pedigrees

• A pedigree is a diagram that shows how a family
  trait is inherited over several generations.
• They are helpful if the trait is a genetic
  disorder and the family members want to know if
  they are carriers.
• Carriers are individuals who are heterozygous for
  an inherited disorder but do not show symptoms of
  the disorder.
• Carriers can pass the allele for the disorder to
  their offspring.

17

• Squares males
• Circles females
• Horizontal lines matings
• Vertical lines indicate offspring (arranged from
  left to right in order of their birth).
• Colored symbols represent affected individuals

18
Autosomal or Sex-linked?

• Autosomal traits will appear in both sexes
  equally. Autosomes are chromosomes other than an
  X or Y sex chromosome.
• Sex-linked traits are located on the X
  chromosome. Most sex-linked traits are recessive.
• Because males have only one x chromosome, a male
  who carries a recessive allele on the X
  chromosome will have the condition.

19

• Autosomal dominant traits are expressed if the
  individual has a parent with the trait.
• If the autosomal trait is recessive, one, both or
  neither of the parents may have the trait.
• Do page 343 Evaluating a pedigree.

20
Autosomal recessive or autosomal dominant?
21
Complexity of Heredity

• Polygenic Traits- several genes influence a
  trait.
• The genes may be scattered along the same
  chromosome or located on a different chromosome.
• Many different combinations appear in offspring.

22

• Intermediate Traits
• Examples include eye color, height, weight and
  hair/skin color.
• All have intermediate conditions between one
  extreme and the other.

Blue eyes
Blue/black eyes
23

• Incomplete dominance
• Intermediate traits- a blending of traits from
  female and male.
• Snapdragons example red flower X white flower
  pink flower

24

• Codominance
• Two dominant traits are expressed at the same
  time.
• A cross between a homozygous red horse and a
  homozygous white horse results in a heterozygous
  offspring with both red and white hairs,
  producing the mixed color roan.

25

• Multiple Alleles
• Genes with 3 or more alleles.
• ABO blood groups (which produces 4 different
  blood types) are determined by 3 alleles IA, IB
  and i.
• IA and IB are dominant over recessive i. When
  both IA and IB are present, they are codominant.

26

• Traits influenced by the environment
• Examples soil acidity determines flower color in
  hydrangeas.
• Arctic foxs fur changes color according to
  changes in the temperature.
• Enzymes make pigments in the summer to darken the
  coat.
• In the winter, no pigment is made so that it can
  blend with the environment.