Genetics

1
Genetics

• The real key to explaining Inheritance

2
Objectives

• To understand the experiments and conclusions of
  Gregor Mendel
• To compare Monohybrid and Dihybrid crosses
• To compare Complete, Incomplete and Co-dominance
• To understand how Multiple Alleles work

3
Gregor Mendel

• Austrian Monk in the 1800s
• Experimented with pea plants
• Kept detailed records of several characteristics
  for generations
• Height
• Flower color
• Pod color
• Shape of peas and pods
• Position of flowers on the plant

4
Mendels Observations

• If he had tall plants in one section of the
  garden, they always produced tall offspring
• Similarly if he had short plants in another
  section the same would occur (short offspring)
• He called these plants Pure Lines
• Tall x Tall Tall
• Short x Short Short

5
Mendels Experiment

• He decided to cross (breed) these pure lines
• Pure Line Tall x Pure Line Short
• The result was ALL tall plants
• He decided to cross these plants with each other
• To his surprise, this resulted in both tall and
  short plants
• The ratio was exactly 3 to 1 (tall to short)

6
Mendels Conclusions(3)

• All characteristics are controlled by factors
  inherited from your parents
• There are two factors for every trait
  (characteristic)
• These traits can be hair color, length of nose,
  skin tone, height, etc.
• One factor can dominate another
• Mask the other
• These factors separate from each other during sex
  cell formation (meiosis)

7
Modernizing Mendels Conclusions

• We now call these factors Alleles
• Alleles can be Dominant or Recessive
• We represent Alleles with letters
• Capitals Dominant Allele
• Lowercase Recessive Allele
• Dominant Alleles always go first
• Genotype is the combination of alleles you have
• Phenotype is the expressed trait
• Homozygous 2 alleles that are the same.
• Heterozygous 2 alleles that are different

8
Types of Genotypes

• Genotypes can also be expressed in words
• Homo Latin prefix for the same

9
The Punnett Square

• He then took those hybrids (Tt) and crossed them
  with one another

• Mendel started his experiments crossing a pure
  line of tall plants (TT) with a pure line of
  short plants (tt)

10
Sample Problem 1

• Widows peak is dominant to straight hairline
• A homozygous woman with widows peak reproduces
  with a man with a homozygous man with straight
  hairline
• What hairline will their children (F1) have?

• What genotype (in words) can we expect from a
  homozygous dominant, homozygous recessive cross?
• ALL offspring will be heterozygous

11
Sample 1 Continued

• Now if an offspring of that cross reproduces with
  another of the same genotype, what hairlines can
  we expect for their children?
• What genotype(s) (in words) can we expect from a
  cross such as this?
• What ratio of phenotype?

12
Our Findings

• Homozygous dominant crossed with homozygous
  recessive yields heterozygous offspring (100)
• A heterozygous crossed with heterozygous yields
  three options
• Homozygous Dominant
• Heterozygous
• Homozygous Recessive
• The ratio of those options are
• A genotype Ratio of 121 (HDHHR)
• A phenotype Ratio of 31 (Dominant Trait
  Recessive)

13
Sample Problem 2

• Attached earlobes are recessive to un-attached
  (hanging) lobes
• A homozygous dominant man (for the trait) mates
  with a heterozygous woman (for same trait)
• What are the chances of their offspring having
  attached earlobes?

• The chances of their child having a child with
  attached earlobes is 0, 0 in 4, 0/4

14
Lets follow Mendels Experiment

• P1
• F1
• F2

15
Lets try some problems

• 1. Genetics Practice Problems
• 2. Bikini Bottom Genetics

16
Variations in Dominance

• All of the eight traits Mendel looked at had two
  alleles (dominant recessive)
• This all-or-nothing expression is known as
  complete dominance
• Another form is incomplete dominance
• This is where neither of the alleles can be
  considered dominant or recessive
• The phenotype is a mixing of the two different
  alleles
• The classic case is with flowers
• Red flowers crossed with white flowers gives us
  pink flowers

17
Incomplete Dominance

• In order to represent these alleles we need to
  make a change
• Since were talking about color well use the
  letter C
• For each color we add the capital superscript to
  the allele
• Capital designation always indicates Incomplete
  Dominance
• Therefore
• CR Red Allele
• CW White Allele
• What the genotypes mean
• CR CR Red
• CW CW White
• CR CW Pink
• CW CR Pink

18
Codominance with Flowers

• With codominance neither allele is dominant, they
  both show through
• Lets look at flower color
• In codominance a plant with red flowers mating
  with one having white flowers produces a plant
  with what?
• Both red white flowers
• So again we write the alleles differently
• Lower case designation indicates Codominance
• For flower color
• Cr Cr Red Flowers
• Cw Cw White Flowers
• Cr Cw Red White Flowers

• Lets cross a homozygous red plant with a
  heterozygous one
• What are the chances of our baby plant having red
  white flowers?

19
Multiple Alleles

• Each allele is dominant to the remaining
• E1 is dominant to E2, E3, E4
• E2 is dominant to E3 E4
• E3 is dominant to E4
• E4 is the recessive allele
• Options for genes would be
• E1 E4 Red Eyes
• E2 E3 Apricot
• E4 E4 White
• Examples of Multiple Alleles
• Blood Types
• Types of Corn
• Feather Color (in some cases)

• Some traits are controlled by multiple alleles
• It still only takes two alleles to make a gene
• However there are more than two options for
  alleles
• For example, the eye color of fruit flies have
  four options for alleles
• E1 Red
• E2 Apricot
• E3 Honey
• E4 White

20
Lets try a Multiple Allele Cross

21
Monohybrids Dihybrids

• Everything we talked about so far have been
  examples of monhybrid crosses
• Monohybrids crosses that are looking at one set
  of alleles
• We can also do dihybrid crosses
• Dihybrids crosses that are looking at two sets
  of alleles
• The following are two examples
• A monohybrid cross
• A dihybrid cross
• The color is just coding for different phenotypes

22
Independent Assortment
23
Dihybrid Example

• In peas plants tall is dominant to short and
  purple flowers are dominant to white
• So if a pea plant is heterozygous for height and
  heterozygous for flower color what is their
  genotype?
• TtPp
• During the creation of sex cells (meiosis) there
  are four combinations of alleles the gamete (sex
  cell) could hold
• TP (Dominant Dominant alleles)
• Tp (Dominant Recessive alleles)
• tP (Recessive Dominant alleles)
• tp (Two recessive alleles)
• If a plant that is heterozygous for both traits
  reproduced with a plant having the same genotype,
  what might the phenotype of their offspring be?

24
Dihybrid Cross Summary

• If we look at the combinations of phenotypes
  possible from a dihybrid cross between two
  heterozygous organisms for two traits we find a
  ratio of 9331
• 9 Tall plants with purple flowers
• 3 Tall Plants with white flowers
• 3 Short Plants with purple flowers
• 1 Short Plant with white flowers

25
Dihybrid Cross Practice

• Dihybrid Cross Problems
• Guinea Pig Activity
• Mendel's Peas and the Pedigree Chart