GENETIC CONCEPTS/PROBLEMS

Be sure to have good working definitions for the genetics vocabulary [Table 13.1, 13.6]

Individuals [parents, offspring, plants, animals, most protists] have paired chromosomes
termed homologous chromosomes.  Homologues have essentially the same length, position of
the centromere,and staining properties;  they also carry the alleles for the same traits.

In simple inheritance,  a single pair of alleles control a trait [such as eye color,
coat color, flower color]. Alleles may be dominant or recessive, although there are
cases when dominance is incomplete or alleles are codominant (see text).

When gametes form (includes meiosis), the sex cells have half the number of chromosomes
(and alleles)as the parent cell.   Humans have 46 chromosomes [diploid]; 
eggs or sperm have 23[haploid].  This haploid number is not arbitrary nor random.  
One member of each homologous pair (23 pairs) is represented in the gamete.

When we use the representation of   Dd   to designate a rabbit which is heterozygous for
coat color, we imply that  D and d are alleles;   D is dominant to d;  D is on one
chromosome and  d is on its homologue.
The phenotype of this rabbit is dark coat [or whatever color is specified].
This animal will produce haploid gametes (many) which contain one allele for the trait:
½ will contain D and   ½  will contain d.   This is Mendel’s Law of Segregation (pg
262-68).

If this animal is mated to one with the same genotype, a monohybrid cross, the expected
phenotypic ratio will be 3 dark to 1 light. 
The genotypic ratio will be 1DD: 2 Dd:1 dd.
This is consistent with the discussion in chpt 13 about Mendel’s peas. 

Do these crosses for practice.  Give the genotypic and phenotypic ratio for each.

1.  Heterozygous dark coat  x   recessive light coat

2.  Homozygous dark coat  x  light coat

3.  Heterozygous  hard shell [H]  x   recessive [h]  soft shell

4.  Heterozygous hard shell  x  heterozygous hard shell
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In dihybrid crosses, organisms with 2 traits are mated.

A homozygous trotter (T)with white (d)coat is crossed with a pacer (t) with a
homozygous dark coat (D).  Write the genotype of each parent.
What offspring will be produced?  Give genotype and phenotype.

The 1st parent has the genotype -  TTdd  
[homozygous for gait and homozygous for white because it is recessive]

The 2nd parent -  ttDD   [same reasoning]

All their offspring will receive a gamete 
[must contain one allele from each pair of alleles] from the 
  1st parent Td  [only choice]  and  2nd parent  tD  [only choice] 
Joining these at fertilization produces offspring which are
        heterozygous for each trait  TtDd
      its phenotype is trotter, dark coat

Another  Mendelian Law  is apparent when looking at the kinds
of gametes produced by an organism which is heterozygous for 2 traits.

     If an organism is RrEe it will produce gametes which must have
        each trait represented [one R and one E] but all possible
        combinations must be considered -

            RE, Re, rE,  re

     This is the Law of Independent Assortment.

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Sex in many animals is inherited through a pair of sex chromosomes. 
  In man, fruit flies and others,  XX designates the female while  XY is the male.

Some traits, other than sex, related may be carried on the sex chromosome, usually the X.
  This is commonly represented by using a superscript.  The X chromosome and its superscript 
  is inherited as a "package" deal.

Autosomal linkage refers to alleles for different traits carried on the same chromosome.
   These are written in parentheses.  Each parenthesis represents one member of the 
    homologous pair of chromosomes.

   For example:  (LMN)(lmn)  represents a pair of homologous chromosomes carrying 
     alleles for 3 traits - L, M, N.  One chromosome carries all 3 dominant alleles
     while its homologue carries the recessive alleles.  
      [This is an arbitrary choice for the example]

     An individual with this genotype can produce two main types of gametes.
       Those containing (LMN) or  containing (lmn).
     If the alleles are far enough apart on the chromosomes, crossing-over may occur
       leading to the formation of recombinant gametes -
        single cross-over   (Lmn) and (lMN)  or  (LMn)(lmN)
        double cross-over   (LmN) and (lMn)

     Draw these out to visualize.
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Try these problems:


In guinea pigs, albino coat is recessive to normal coat color (N).
      Long fur is recessive to short fur (F).

      An animal which is heterozygous for both traits
                is mated to another which is
        heterozygous for coat color and has long fur.

      What is the expected phenotypic ratio of the offspring? 


Tall [T] is dominant to short.
When tall plants are crossed with short plants, the offspring
 are   50% tall and 50%  short
Give the genotype of both parents.
 

List all kinds of gametes produced by an individual with the genotype -

       qqWW(FG)(FG)XX


Write the genotype for a woman who is heterozygous for the sex-linked trait
   called hemophilia.  H is normal blood, h represents hemophilia.


Given the genotype (UPS)(ups)- list parental gametes and recombinant gametes.


Link to Review for Test #3b
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