Introduction to Genetics

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Introduction to Genetics

• Chapter 11

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What is genetics?

• The scientific study of heredity

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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

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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

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Mendels Peas

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

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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

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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.

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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

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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.

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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.

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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?

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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)

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Probability

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

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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.

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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.

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Punnett square example
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genotypes

• Homozygous when an organism possesses two
  identical alleles. ex.
• YY or yy
• Heterozygous when an organism possesses
  different alleles. ex.
• Yy

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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.

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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.

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Punnett square review
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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.

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Some exceptions to Mendels principles

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

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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.

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Japanese four-o-clock flowers

• Red flower plant genotype RR
• White flower plant genotype WW
• Pink flower plant genotype RW

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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

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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)

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Roan cattle inheritance
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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.

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Human Blood types

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

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

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

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

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6 different genotypes

• IAIA
• IAIB
• IBIB
• IBi
• IAi
• i i

• Type A
• Type AB
• Type B
• Type B
• Type A
• Type O

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How common are the different blood types?
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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?

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Polygenic traits

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

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Rediscovery of Mendels work

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

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Thomas Hunt Morgan

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

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Why the Fruit Fly?

1. Can work with large numbers of flies easily
2. Produce many offspring
3. Short reproductive cycle
4. Only four pairs of chromosomes

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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

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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

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Events of meiosis I

• During prophase I, each chromosome pairs with its
  homologous chromosome to form a tetrad

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Crossing-over

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

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Meiosis II

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

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Gametogenesis

• Literally means creation of gametes
• Egg and sperm

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2 types Spermatogeneis Oogenesis
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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.

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Gene Maps

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