Title: The Work of Gregor Mendel
1The Work of Gregor Mendel
2What is genetics? Why study it?
- Genetics study of heredity
- Heredity passing on of characteristics from
parents to offspring - Geneticists study genes, stretches of DNA, that
code for proteins
3What are some features in humans that everyone
has in common?
4What are some features that are different from
human to human?
5Why pea plants?
- Easy to grow and mature quickly
- Structure method of pollination made them easy
to use in controlled experiments could
self-fertilize or cross-fertilize - Different plants show contrasting traits
- Characteristic heritable feature that varies
among individuals, ex flower color - Trait different varieties for a characteristic,
ex purple or white flowers
6- Why did Mendel Choose the Pea?
- Flower Structure/ allowed controlled mating
- Many variable traits/ simple traits
- Fast Generation Time
7Gregor Mendel
- Genetics the scientific study of heredity
- Gregor Mendel (born in 1822) worked with true
breeding garden peas to study inheritance - True breeding produce offspring identical to
themselves when they self-pollinate
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9Genes and Dominance
- Mendel crossed plants with seven different pea
plant traits that each had two contrasting
characteristics and studied the offspring. - P generation original pair of plants
- F1 generation 1st generation of offspring
- F2 generation 2nd generation of offspring
10Mendels Crosses
- Self-pollination sperm cells fertilize egg
cells of the same plant (i.e. one parent, but
still sexual reproduction) - True-breeding plants if allowed to
self-pollinate, they would always produce
offspring identical to themselves - Ex true-breeding short plants always have
offspring that are short when then self-pollinate
11The F1 Cross
- Mendel wondered if the recessive alleles had
disappeared or if they still existed in the F1
generation. - He let the F1 plants self-pollinate creating the
F2 generation. -
- Traits controlled by the recessive alleles
reappeared in 1/4th of the F2 generation.
12Mendel made two conclusions
- Inheritance is determined by factors passed on
from one generation to the next. - Genes pieces of DNA that determine a trait
- Alleles different forms of genes
- Some alleles are dominant and others are
recessive. (Principle of Dominance) - Dominant allele always expressed as a trait
when present (represented by a capital letter) - Recessive allele only expressed when the
dominant allele is not present (represented by a
lower case letter)
13Current genetic terminology
- Phenotype is the outward appearance ex tall or
short height - Memory trick
- Genotype is the genes
- Phenotype is the physical feature you see
14General Terms
- Homozygous having two identical alleles for a
trait (ex TT homozygous dominant or tt
homozygous recessive) - Heterozygous having two different alleles for a
trait (ex Tt) - Hybrid offspring of crosses with different
traits - Genotype genetic makeup of an individual (ex
TT, Tt or tt) - Phenotype physical appearance of an individual
(ex brown, tall) - Probability the likelihood that an event will
occur
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16The Albino Trait
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19Genetics and Probability
- Mendel realized that probability could be used to
explain the results of genetic crosses. - Punnett Squares
-
- T tall 25 TT tall
- 50 Tt tall
- t short
25 tt short - Possible offspring of a Tt x Tt cross
75 chance - tall and 25 - short
(31 ratio)
20Mendels Law of Segregation As seen in P and F1
generations
Dad
Mom
T tall (dominant) TT x tt t
short (recessive)
Parental!
meiosis
Gametes?
21Mendels Law of Segregation As seen in P and F1
generations
Dad
Mom
T tall (dominant) TT x tt t
short (recessive)
Parental!
meiosis
Parent 1 Gametes
Gametes
T t tall T t tall
T t tall T t tall
Parent 2 Gametes
22Mendels Law of Segregation As seen in P and F1
generations
Dad
Mom
T tall (dominant) Tt x Tt t short
(recessive)
T T tall T t tall
T t tall t t short
T
Probabilities?
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24Practicing with Punnett Squares
- Parents TT and Tt
- T T
- T TT TT
-
- t Tt Tt
- Offspring
- 100 TT or Tt tall
- ratio - (40)
- Parents Tt and tt
- T t
- t Tt tt
-
- t Tt tt
- Offspring
- 50 Tt tall
- 50 tt short
- ratio - (22) or (11)
25Beyond Dominant Recessive
- Some genes are neither dominant nor recessive
and many traits are controlled by multiple
alleles or multiple genes. - Incomplete Dominance neither allele is
completely dominant so the heterozygous phenotype
is a blending of traits - Ex four oclocks - red flower crossed
- with white flower produces pink flower
26Codominance
- Q What does cooperate mean?
- A Operate together
- Codominance when both alleles are expressed
neither is dominant nor recessive - Q If the two homozygotes are red and white,
whats the phenotype of the heterozygote? - A The heterozygote shows red and white hairs
- Roan coat in cows and horses
- Both red and white hairs are present
27- Codominance both alleles are dominant so both
traits show in the heterozygous phenotype - Ex chickens white chicken crossed with black
chicken produces black and white speckled chicken - Multiple Alleles many genes have three or more
alleles of the same gene - Ex blood type
- Polygenic traits traits that are controlled by
two or more genes show a wide range of
phenotypes. - Ex skin color is controlled by at least 4 genes
28Sex Linked Traits
- Q What are the two types of chromosomes?
- A Sex chromosomes and autosomes
- Q How many sex chromosomes are there? What are
they? What do the different combinations mean? - A Two sex chromosomes X Y
- XX female
- XY male
29Color-blindness test!
30Color-blindness test!