Mendelian Genetics - cashmerebiology

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					Mendelian Genetics
     Year 13 Biology
Revision – The Basics
n   Monohybrid Cross
    n   Genetic cross that considers only one
Revision – The basics
n   Back-cross or test cross
    n   By looking at the phenotype, you can’t tell if an organism’s
        genotype is homozygous or heterozygous – do a test
        cross by mating your unknown genotype with an organism
        showing the recessive trait.
    n   If any of the offspring show the recessive trait, the original
        unknown parent was heterozygous.
    n   If no recessive trait shown up in the offspring then, it was
Revision – The basics
n       Dominance
    –    Incomplete dominance – when one allele does
         not completely dominate over the other e.g.
         snapdragon flower. When a red flower plant is
         crossed with a white flower plant, the
         heterozygous plant is pink.
    –    Codominance – same as above except the
         heterozygote shows both traits.
Revision – The basics
n    Dominance
    3.   Overdominance – e.g. if one homozygote is tall
         and the other is short then the heterozygote will
         be even taller than the tall homozygote. Can
         lead to hybrid vigour.
Revision – The basics
n   Lethal Genes
    n   Homozygous dominant is lethal (dies as an
    n   Instead of 1:2:1 ratio you get a 2:1 ratio

    n   E.g. In mice yellow fur is dominant to grey. If two
        yellow mice mate you get ¼ YY (died), ½ Yy
        (yellow), ¼ yy (grey) – the homozygous dominant
        is lethal.
Revision – the basics
n   Multiple Alleles
    n   Three possible alleles for each gene.
    n   E.g. Blood groups – A, B and O
    n   A and B are codominant and they are both dominant
        to O, which is recessive.
         Blood Group   Genotype      Antigen on Cell   Serum agglutinin

         A             IAIA or IAi   A                 Anti-B

         B             IBIB or IBi   B                 Anti-A

         AB            IAIB          A and B           None

         O             ii            none              Anti-A and anti-B
Revision – the basics
n   Dihybrid Cross
    n   Cross between organisms with two different
    n   Remember:
         n   Phenotype ratios for a dihybrid hetetozygous
             cross 9:3:3:1
         n   9/16 – the two dominant alleles
         n   3/16 – one dominant + one recessive
         n   3/16 – other dominant + other recessive
         n   1/16 – the two recessives
Gene-Gene Interactions
n    Situations where genes on two different
     loci or on different chromosomes have an
     affect on each other.
    1. Collaboration

    2. Complementary genes

    3. Supplementary genes (epistasis)
n   When one characteristic is
    controlled by two or more
    pairs of alleles.
n   Combinations of dominant
    and recessive alleles             Rose    Pea
    result in up to four different
n   E.g. comb shape in
    9:3:3:1 phenotype ratio          Walnut   Single
n   See page 132 biozone for the cross

9/16 P_R_ two dominants = Walnut
3/16 P_rr one dominant + one recessive = Pea
3/16 ppR_ other dominant +other recessive = Rose
1/16 pprr two recessives = Single
Complementary Genes
n   Where one dominant allele can only be
    expressed in the presence of another dominant
    allele (of another gene pair)
n   E.g. white and purple flower colours – must
    have both P and C alleles for purple.
    n   9:7 phenotype ratio
    n   Only two “visible” phenotypes
    n   See page 135 biozone for full cross
    n   To make purple pigment, both P and C are needed i.e.
        they are complementary
Supplementary Genes (epistasis)
n   Where one particular allele (out of 4 alleles) must be present
    for the full expression of another allele.
n   E.g. mouse fur colour

    _ _ cc – albino
    _ _ _ C – Brown
    B_ C_ - black

n   Allele C supplements allele B
n   9:3:4 phenotype ratio – only 3 “visible” phenotypes
n   See page 136 biozone for full cross.
  Ratios you should know
Offspring Ratio      Parents’ genotype   Type of inheritance
Monohybrid crosses
3:1                  Aa X Aa             A dominant to a
1:2:1                Aa X Aa             Incomplete or
1:1                  Aa X aa             Back or test-cross
2:1                  Aa X Aa             Lethal genes (AA dies)
Dihybrid Crosses
9:3:3:1              AaBb X AaBb         Heterozygous cross
1:1:1:1              AaBb X aabb         Back or test cross
9:3:4                AaCc X AaCc         Supplementary (epistasis)
9:7                  AaCc X AaCc         Complementary

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