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Introduction to Genetics

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Chapter 11 What is genetics? The scientific study of heredity Gregor Mendel Born in 1822 in Czechoslovakia. Became a monk at a monastery in 1843. – PowerPoint PPT presentation

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Title: Introduction to Genetics


1
Introduction to Genetics
  • Chapter 11

2
What is genetics?
  • The scientific study of heredity

3
Gregor Mendel
  • Born in 1822 in Czechoslovakia.
  • Became a monk at a monastery in 1843.
  • Taught biology and had interests in statistics.
  • Also studied at the University of Vienna

4
Mendel continued
  • After returning to the monastery he continued to
    teach and worked in the garden.
  • Between 1856 and 1863 he grew and tested over
    28,000 pea plants

5
Mendels Peas
  • Easy to grow.
  • Easily identifiable traits
  • Can work with large numbers of samples

6
Mendels experiments
  • The first thing Mendel did was create a pure
    generation or true-breeding generation.
  • He made sure that certain pea plants were only
    able to self pollinate, eliminating unwanted
    traits.
  • He did this by cutting away the stamen, or male
    part of each flower

7
Genes and dominance
  • Trait a characteristic
  • Mendel studied seven of these traits
  • After Mendel ensured that his true-breeding
    generation was pure, he then crossed plants
    showing contrasting traits.
  • He called the offspring the F1 generation or
    first filial.

8
What will happen when pure yellow peas are
crossed with pure green peas?
  • All of the offspring were yellow.
  • Hybrids the offspring of crosses between
    parents with contrasting traits

9
What did Mendel conclude?
  • Inheritance is determined by factors passed on
    from one generation to another.
  • Mendel knew nothing about chromosomes, genes, or
    DNA. Why?
  • These terms hadnt yet been defined.

10
What were Mendels factors
  • The factors that Mendel mentioned were the
    genes.
  • Each gene has different forms called alleles
  • Mendels second principle stated that some
    alleles are dominant and some are recessive.

11
Mendels second cross
  • He allowed the F1 generation to self-pollinate
    thus producing the F2 generation.
  • Did the recessive allele completely disappear?
  • What happened when he crossed two yellow pea
    hybrid (F1) plants?

12
Results
  • ¾ of the peas were yellow, ¼ of the peas were
    green.
  • During the formation of the sex cells or gametes,
    the alleles separated or segregated to different
    gametes. (pollen and egg)

13
Probability
  • The likelihood of a particular event occurring.
    Chance
  • Can be expressed as a fraction or a percent.
  • Example coin flip.

14
Punnett Square
  • Developed by Reginald Punnett.
  • A diagram used to show the probability or chances
    of a certain trait being passed from one
    generation to another.

15
Reading Punnett squares
  • Gametes are placed above and to the left of the
    square
  • Offspring are placed in the square.
  • Capital letters (Y) represent dominant alleles.
  • Lower case letters (y) represent recessive
    alleles.

16
Punnett square example
17
genotypes
  • Homozygous when an organism possesses two
    identical alleles. ex.
  • YY or yy
  • Heterozygous when an organism possesses
    different alleles. ex.
  • Yy

18
Phenotype vs genotype
  • Genotype
  • The genetic makeup
  • Symbolized with letters
  • Tt or TT
  • Phenotype
  • Physical appearance of the organism
  • Expression of the trait
  • Short, tall, yellow, smooth, etc.

19
Probability and statistics
  • No one event has a greater chance of occurring
    than another.
  • You cannot predict the precise outcome of an
    individual event.
  • The more trials performed, the closer the actual
    results to the expected outcomes.

20
Punnett square review
21
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22
Independent Assortment
  • The two factor cross. Example color and shape of
    peas.
  • F1 cross to produce the F2 generation
  • Ex RRYY x rryy
  • Round yellow mated with wrinkled green
  • Offspring would all be hybrid for both traits
    (RrYy)

23
What is independent assortment?
  • Alleles separate independently during the
    formation of gametes.

24
The dihybrid cross
  • Punnett square on board

25
Mendels death
  • Mendel published his paper on heredity in 1866.
  • The scientific community saw little if any
    importance in his work.
  • Mendel died in 1884 with no recognition for his
    contributions to genetics.

26
Some exceptions to Mendels principles
  • Some alleles are neither dominant nor recessive.
  • Many traits are controlled by more than one gene
    (polygenic traits)

27
Incomplete dominance
  • A situation in which neither allele is dominant.
  • When both alleles are present a new phenotype
    appears that is a blend of each allele.
  • Alleles will be represented by capital letters
    only.

28
Japanese four-o-clock flowers
  • Red flower plant genotype RR
  • White flower plant genotype WW
  • Pink flower plant genotype RW

29
What happens when a red flower is crossed with a
white flower?
  • According to Mendel either some white and some
    red or all offspring either red or white.
  • All are pink

30
Codominance
  • When two alleles both appear in the phenotype.
  • Usually signified using superscripts.
  • example color of hair coat in cattle.
  • crcr red hairs
  • cwcw white hairs
  • crcw roan coat (mixture of both colors)

31
Roan cattle inheritance
32
Multiple allele inheritance
  • When two or more alleles contribute to the
    phenotype.
  • Human blood types A,B,O and AB
  • A and B are codominant to each other.
  • Both A and B are dominant over O.

33
Human Blood types
  • TYPE A
  • Allele IA
  • Blood cells have small antigens on the surface.

34
  • TYPE B
  • Allele IB
  • Cells coated with type B antigens

35
  • TYPE AB
  • genotype IAIB
  • Blood cells contain both types of antigens
  • Known as universal recipient

36
  • TYPE O
  • Allele i
  • No antigens on the surface of the blood cells
  • Known as universal donor

37
6 different genotypes
  • IAIA
  • IAIB
  • IBIB
  • IBi
  • IAi
  • i i
  • Type A
  • Type AB
  • Type B
  • Type B
  • Type A
  • Type O

38
How common are the different blood types?
45
40
4
11
39
Sample Problem
  • A man with type AB blood marries a woman with
    type B blood whose father has type O blood. What
    are the chances that they have a child with type
    A blood? Type AB?

40
Polygenic traits
  • Traits controlled by two or more genes.
  • Examples
  • Human height,
  • eye and skin
  • color

41
Rediscovery of Mendels work
  • Around the turn of the century (early 1900s)
    many scientists rediscovered Mendels work
  • 1908 Garrod
  • 1902 Sutton
  • 1910 Morgan

42
Thomas Hunt Morgan
  • 1866-1945
  • Born in Kentucky, professor of Biology at
    Columbia U.
  • Worked with fruit flies (drosophila)
  • Nobel Prize in Medicine (1933)

43
Why the Fruit Fly?
  • Can work with large numbers of flies easily
  • Produce many offspring
  • Short reproductive cycle
  • Only four pairs of chromosomes

44
Meiosis
  • A method of cell division similar to mitosis.
  • 2 main differences
  • There are two divisions to produce 4 daughter
    cells
  • The cells produce contain ½ the chromosomes as
    the original cell

45
Chromosome number
  • All cells of an organism contain a specific
    number of chromosomes.
  • Most cells are diploid (2n) meaning they have two
    copies of each chromosome

46
Events of meiosis I
  • During prophase I, each chromosome pairs with its
    homologous chromosome to form a tetrad

47
Crossing-over
  • Crossing-over an exchange of genetic material
    between sister chromatids
  • Results in greater variation

48
Meiosis II
  • Neither cell replicates its chromosomes.
  • Each cell splits (similar to mitosis)
  • Produces four daughter cells.
  • Animation

49
Gametogenesis
  • Literally means creation of gametes
  • Egg and sperm

50
2 types Spermatogeneis Oogenesis
51
Net result
  • Spermatogensis
  • 4 mature sperm
  • Each sperm has exactly half the number of
    chromosomes as the father.
  • Oogensis
  • 1 mature ova or egg.
  • Each egg has exactly half the number of
    chromosomes as the mother.

52
Gene Linkage
  • Are genes linked to each other on chromosomes?
  • Morgan found that many genes are linked together.
  • It was determined that chromosomes, not genes,
    assort independently during meiosis.

53
Gene Maps
  • First developed by Sturtevant in 1911.
  • The farther apart two genes are, the more likely
    they will be separated in meiosis.

54
Assignment
  • worksheet
  • Pages 283-284
  • 1-10, 13, 14, 17, 18, 24
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