Download Heredity and Monohybrid Crosses - Student Note

HEREDITY AND
MONOHYBRID CROSSES
Mendel’s Experiments
Homozygous/Heterozygous
Genotype/Phenotype
Dominant/Recessive
Incomplete Dominance/Codominance
Punnett Squares
GENETICS & HEREDITY

genetics


study of heredity and the variations of inherited
traits
heredity

passing of traits from parents to offspring
GENETICS BEFORE MENDEL

some scientists believed that traits from parents
are blended together to produce offspring


ex – a tall parent and a short parent would produce
an average child
observation allowed other scientists to notice that
offspring sometimes exhibited traits identical to
that of only one parent

ex – a brown eye parent and a blue eye parent
produce a brown eye child
GREGOR MENDEL
1822-1884
 monk
 father of genetics
 studied pea plants to
determine how traits
where passed on from
one generation to the
next

http://www.youtube.com/watch?v=aDpLDBaEBjk&feature=related
WHY USE PEA PLANTS?
flowers have both male and
female reproductive organs
 plants can be self pollinated or
cross pollinated
 have many traits that can be
observed

MENDEL’S EXPERIMENT
crossed/mated two true breading plants (plants
that produce offspring that are genetically
identical to the parent)
 when P generation (parent plants) plants were
crosses all offpsring (F1 generation) displayed the
same trait
 when two F1 generation plants were crosses to
produced offspring (F2 generation) traits were
repeated in a ratio of 3:1

MENDEL’S EXPERIMENT
http://www.youtube.com/watch?v=xrDjN_4HKf0&feature=related
MENDEL’S CONCLUSIONS

Law of Segregation
for each characteristic (trait) and organism carries
two factors (genes), one from each parent
 parent organisms donate only one copy of each factor
(gene) in their gametes (sex cells)
 during meiosis the two factors (genes) are separated

HOMOZYGOUS/HETEROZYGOUS

homozygous

individual that carries two of the same alleles for a
given characteristic/trait


ex – two blue eyed genes (bb), two brown eyed genes (BB)
heterozygous

individual that carries two different alleles for a
given characteristic/trait

ex – one blue eyed gene and one brown eyed gene (Bb)
GENOTYPE/PHENOTYPE

genotype

genetic makeup of an individual


ex – an individual carries one blue eyed gene and one brown
eyed gene (Bb)
phenotype

outward appearance of the individual

ex – an individual has brown eyes
DOMINANT/RECESSIVE

dominant allele
allele that is always expressed (seen) if it is present
 represented by a capital letter in genotype



ex – brown eye allele (B)
recessive allele
allele that is expressed only if the dominant allele is
not present
 individual must be homozygous for the recessive
allele
 represented by a lower case letter in genotype


ex – blue eye allele (b)
COMPLETE AND INCOMPLETE DOMINANCE

complete dominance
only one allele is expressed
 one allele determines the
phenotype of the individual


incomplete dominance
neither allele is dominant over
the other
 both alleles are expressed
 phenotype of the individual is
a result of both alleles

CODOMINANCE
both alleles are expressed fully
 offspring with both alleles show a third
phenotype


ex – red bull and white cow produce roan calves (have
both pure white hair and pure red hair)
BLOOD TYPES – CODOMINANCE AND DOMINANCE





blood types show both codominance and dominace
three alleles are present
 type A allele – IA
 type B allele - IB
 type O allele - i
type A and B blood alleles are codominant (both are
expressed if an individual carries both)
type A blood alleles are dominant over type O blood alleles
(individual will express type A blood)
type B blood alleles are dominant over type O blood alleles
(individual will express type B blood)
BLOOD TYPES – CODOMINANCE AND DOMINANCE
BLOOD TYPES – CODOMINANCE AND DOMINANCE
PUNNETT SQUARE QUESTIONS
diagram used to predict genotype and phenotype
of offspring
 dominant genes are represented by a capital
letter and recessive genes are represented by a
lowercase letter

Example 1
In pea plants, the allele for yellow seed colour, Y, is dominant over that for green seed colour, y.
Consider a cross between a pea plant that is homozygous for yellow seeds and a plant that is
homozygous for green seeds. Determine the possible genotypes and phenotypes of the offspring.
PUNNETT SQUARE QUESTIONS
Example 2
Two heterozygous yellow seed plants (Yy) are crossed. Determine the genotype and phenotype of the
offspring.
PUNNETT SQUARE QUESTIONS
Example 4
A researcher crossed a homozygous yellow seed plant (YY) and a heterozygous yellow seed plant (Yy).
Determine the genotype and phenotype ratios of the offspring.
PUNNETT SQUARE QUESTIONS
Example 5
A farmer crossed a heterozygous yellow seed plant with a recessive green seed plant. Determine the
genotype and phenotype of the offspring.
PUNNETT SQUARE QUESTIONS

incomplete dominance and codominance are
represented by a capital letter and a superscript
letter
Example 6
Snapdragons are coloured based on two alleles that show incomplete dominance, white (CW) and red
(CR). What are the genotypes and phenotypes of the F1 and F2 generations?
PUNNETT SQUARE QUESTIONS
Example 7
William and Kate have three children. Their blood types are AB, O and A. What are the genotypes of
each child? What are the genotypes and phenotypes of William and Kate?