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					Gregor Mendel and his garden
Mendel‟s garden pea plant
    What you see -phenotype
Which alleles are involved- genotype
The seven traits that
Gregor Mendel studied

Each one showed a pattern of
a dominant and recessive allele

For each characteristic,
it was either one or the other,
never a blending of the traits.
The Law of

Segregation results from
the formation of
by meiosis.

The hybrid parents pass
on only one of their two

The offspring receive
one allele from each
parent for each gene.
A dihybrid cross:

A dihybrid cross in parakeets
     Homologous chromosomes
have information about the same traits
What’s the chromosomal basis of inheritance?
            Meiosis helps explain
     the Law of independent assortment
We have seen the dominant and recessive pattern that Mendel
    discovered…..but not all of genetics is “Mendelian”
            1. Incomplete Dominance

                 CRCR                    CWCW

                 CRCW                    CRCW

          CRCR          CRCW      CRCW          CWCW

•Follows rules of Mendelian genetics and segregation
•Neither allele for the flower color gene dominates the other
Non-Mendelian inheritance
For some traits, there are „carriers,‟
 They do not show the phenotype
A pedigree chart for a recessive trait -
only the pure recessive shows the trait
  why do John and Hepzibah have question marks?
           A pedigree chart for a recessive trait -
           only the pure recessive shows the trait
              why do John and Hepzibah have question marks?

Make a simple
pedigree for your own
family (aunts and
uncles and cousins)

 Square = male
 Circle = female
 Straight line
connects parents
 Siblings hang down
from line of children
Pleiotropy - one allele has several phenotypical effects
          Genes with multiple alleles

                                      White with
  Brown               Chinchila       black tips         White

   1.C C
                       1.cch cch ch
   2.C c                                                   1.c c
                       2.cch ch         2.c hc
   3.C ch              3.cch c
   4.C c
•Mendelian ideas still work with four different alleles for
coat color!
The ABO blood groups
 three alleles - four blood types
     Environmental effects on phenotype

•Siamese Cats have a gene [C] that determines coat color
•The C gene causes dark colorations in the colder parts of the cat‟s
•The environment affects the phenotype of the cat by causing
expression of a gene, but the genotype is unaffected
                           Practice problems
1.   Black coat color in Cocker Spaniels is governed by a dominant allele B and
     red coat color is governed by its recessive allele b; solid pattern is governed
     by the dominant allele of an independently assorting locus S, and spotted
     pattern by its recessive allele s. A solid-black male is mated to a solid-red
     female and produces a litter of 6 pups: 2 solid black, 2 solid red, 1 black
     and white, and 1 red and white. Determine the genotypes of the parents.
2.   Aliens are attacking the earth and settling in the Bay Area. As a budding
     geneticist you are interested to find that they have two traits each
     determined by single independently segregating genes. When the aliens
     have the dominant H allele they have very elastic heads, while the recessive
     h allele gives a very rigid head. Additionally, small brain size is due to a
     dominant allele B and large brain size is determined by a recessive allele b.
     When two parents heterozygous for brain size and head elasticity mate they
     produce: 9 small brained elastic headed offspring, 3 large brained elastic
     headed offspring, and 3 small brained rigid headed offspring. Do these
     genes follow the laws of Mendelian segregation? How? Which class is

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