GENETIC INHERITANCE

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GENETIC INHERITANCE
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Lesson Objectives

• At the end of this lesson you should be able to
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• Give a definition for a gamete
• Understand gamete formation
• Give the function of gamete in sexual
  reproduction
• Define fertilisation
• Define allele
• Differentiate between the terms homozygous and
  heterozygous

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Lesson Objectives (cont.)

• At the end of this lesson you should be able to
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• Differentiate between genotype and phenotype
• Differentiate between dominant and recessive
• Show the inheritance to the F1 generation in a
  cross involving
• Homozygous parents
• Heterozygous parents
• Sex determination
• Show the genotypes of parents, gametes and
  offspring

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Sexual Reproduction

• Involves two parents
• Each parent makes reproductive cells
• - called gametes

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Outline of Fertilisation

• Gametes join together by fertilisation
• Form a diploid zygote
• This develops into an embryo
• Eventually into a new individual
• New individual resembles both parents but is
  not identical to either

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What are Gametes?

• Reproductive Cells
• Formed by meiosis
• Contain single sets of chromosomes
• - haploid
• Capable of fusion to form zygote
• - diploid
• Zygote contains genetic information of both
  gametes

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Sex Determination
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Human Chromosomes

• We have 46 chromosomes, or 23 pairs.
• 44 of them are called autosomes and are numbered
  1 through 22. Chromosome 1 is the longest, 22 is
  the shortest.
• The other 2 chromosomes are the sex chromosomes
  the X chromosome and the Y chromosome.
• Males have and X and a Y females have 2 Xs XY
  vs. XX.

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Male Karyotype
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Female Karyotype
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Sex Determination

• The basic rule
• If the Y chromosome is present, the person is
  male.
• If absent, the person is female.

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Meiosis

• the X and Y chromosomes separate and go into
  different sperm cells
• ½ the sperm carry the X and the other half carry
  the Y.
• All eggs have one of the mothers X chromosomes
• The Y chromosome has the main sex-determining
  gene on it, called SRY

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Sex Determination

• About 4 weeks after fertilization, an embryo that
  contains the SRY gene develops testes, the
  primary male sex organ.
• The testes secrete the hormone testosterone.
• Testosterone signals the other cells of the
  embryo to develop in the male pattern.

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Genetics

• The study of heredity.
• Gregor Mendel (1860s) discovered the fundamental
  principles of genetics by breeding garden peas.

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Genetic Terms - Alleles

• Alternative forms of genes.
• Units that determine heritable traits.
• Dominant alleles (TT - tall pea plants)
• a. homozygous dominant
• Recessive alleles (tt - dwarf pea plants)
• a. homozygous recessive
• Heterozygous (Tt - tall pea plants)

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Phenotype

• Outward appearance
• Physical characteristics
• Examples
• 1. tall pea plant
• 2. dwarf pea plant

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Genotype

• Arrangement of genes that produces the phenotype
• Example
• 1. tall pea plant
• TT tall (homozygous dominant)
• 2. dwarf pea plant
• tt dwarf (homozygous recessive)
• 3. tall pea plant
• Tt tall (heterozygous)

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Punnett square

• A Punnett square is used to show the possible
  combinations of gametes.

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Breed the P generation

• tall (TT) vs. dwarf (tt) pea plants

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tall (TT) vs. dwarf (tt) pea plants
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Breed the F1 generation

• tall (Tt) vs. tall (Tt) pea plants

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tall (Tt) vs. tall (Tt) pea plants
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Monohybrid Cross

• A breeding experiment that tracks the inheritance
  of a single trait.
• Mendels principle of segregation
• a. pairs of genes separate during gamete
  formation (meiosis).
• b. the fusion of gametes at fertilization pairs
  genes once again.

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Homologous Chromosomes
This person would have brown eyes (Bb)
Paternal
Maternal
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Meiosis - eye color
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Monohybrid Cross

• Example Cross between two heterozygotes for
  brown eyes (Bb)
• BB brown eyes
• Bb brown eyes
• bb blue eyes

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Monohybrid Cross
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Dihybrid

• A genetic cross where two contrasting traits are
  investigated
• Eg TtYy or TTYY

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Law of Independent Assortment (mendels 2nd law)

• When gametes are formed, each member of a pair of
  alleles may combine randomly with either of
  another pair (if genes are not linked)

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• The allele for tongue rolling (R) is dominant to
  the allele for non tongue rolling (r). Also the
  allele for brown hair (B) is dominant to red hair
  (b). Neither of these characteristics is sex
  linked.
• Using the punnet square determine the possible F1
  generation genotypes of a cross between two
  heterozygous parents (heterozygous for both
  characteristics).

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• In the fruit fly, Drosophila, the allele for grey
  body (G) is dominant to the allele for ebony body
  (g) and the allele for long wings (L) is dominant
  to the allele for vestigial wings (l). These two
  pairs of alleles are located on different
  chromosome pairs.
• (i) Determine all the possible genotypes and
  phenotypes of the progeny of the following cross
  grey body, long wings (heterozygous for both) X
  ebony body, vestigial wings.

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Incomplete Dominance

• Niether genes are dominant over the other and
  form an intermediate phenotype in the
  heterozygous offspring.
• Example snapdragons (flower)
• red (RR) x white (rr)
• RR red flower
• rr white flower

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Incomplete Dominance
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Pink Flowers?
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Co-dominance

• Both alleles are dominant and both express their
  phenotype (multiple alleles) in heterozygous
  individuals.
• Example blood
• 1. type A IAIA or IAi
• 2. type B IBIB or IBi
• 3. type AB IAIB
• 4. type O ii

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Co-dominance

• Example homozygous male B (IBIB)
• x heterozygous female A (IAi)

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Practice with Crosses

• http//www.zerobio.com/drag_gr11/mono.htm

http//www.brooklyn.cuny.edu/bc/ahp/MGInv/MGI.Intr
o.html
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Chromosomes and Genetics

• Chromosomes are long pieces of DNA, with
  supporting proteins
• Genes are short regions of this DNA that hold
  the information needed to build and maintain the
  body
• Genes have fixed locations each gene is in a
  particular place on a particular chromosome
• Diploids have 2 copies of each chromosome,
  one from each parent. This means 2 copies of
  each gene.

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Linkage

• genes are genes located on the same chromosome,
  which tend to be inherited together.

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Example Drosophila fruit fly

• Genes for body colour and wing length are on the
  one chromosome i.e. are linked.
• Grey body (G) and long wings (L) are dominant to
  black body (g) and vestigial wings (l). G with L
  and g with l.  
• Parents GGLL X ggll

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• G G g g
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• L L l l
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• Gametes GL X gl
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• G g
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• L l
• F1 GgLl
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Self-cross (if genes linked)

• Parents GgLl X GgLl
• Gametes GL gl GL gl
• F2 GGLL GgLl GgLl ggll
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• Sex determination
• Autosome a chromosome other than the sex
  chromosomes.
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• Sex chromosomes chromosomes that determine the
  sex of an individual - XX or XY.
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• Parents Mother X Father
• X X X Y
• Gametes X (Egg) X orY (Sperm)
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• F1 genotype XX XY
• Phenotype Female Male

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• The male thus determines the sex of an offspring.
• Mother gives an X to everyone but father gives an
  X or Y chromosome. There is a 5050 chance that
  any child will be male/female)

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• In man sex-linked genes (i.e. those on the X
  chromosome with no corresponding part on the Y
  chromosome) include those governing red-green
  colour blindness, muscular dystrophy and
  haemophilia (inability to clot blood).
• Females with both recessive genes for haemophilia
  do not survive beyond the first four months of
  gestation period.

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• Parents Female carrier X Male normal
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• XHXh XHY
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• Gametes XH Xh XH Y
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• F1 XHXH XHY XHXh XhY
• Female Male Female Male
• Normal Normal Carrier Haemophili
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• 25 chance of producing a haemophiliac child
• 50 chance of producing a haemophiliac son.
• It is the mother that determines if the son is
  haemophiliac or not since the father always
  passes the Y chromosome to his son.

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• Colour blindness is caused by a recessive gene on
  the X chromosome.
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• Parents
• Female carrier X Male colour blind

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End