Transcript Heredity - Science-with

Do Now
If Mary’s dad developed male
patterned baldness, but she did not
have the trait, and she married Joe,
who does not have male pattern
baldness, what would be their
chances of having a son with male
pattern baldness?
Total
marks 4: 1 mark for ratio value, 3 marks for punnett squarea
Since Mary’s dad had the trait but she did
not, it must be sex-linked recessive
XB
XB
Y
XB XB
XBY
Xb
XBXb
XbY
If Mary and Joe
had a baby, it
would have a
25% chance of
being a male
with male pattern
baldness
Multiple Alleles
18.4
Page 608
Beyond Mendel
Multiple Alleles
 Trifolium repens (Clover)
• thus far there has been only three types of genotypes
(homozygous recessive or dominant and heterozygous)
• but in Clover one gene is responsible for all the
patterns on the leaves.
• in most organisms many genes have more than two
alleles.
• a gene with more than two alleles is said to have
multiple alleles.
Beyond Mendel
Polygenic Inheritance
 Mendel selected characteristics that were distinct so there would be no
question of phenotypes.
• since then people have looked at continuous traits
• traits that gradually change from one extreme to another.
• examples: ears, length in corn, weight of beans
• continuous traits are usually controlled by more than one gene.
• traits that are controlled by many genes are called polygenetic
traits.
• a group of genes that all contribute to the same trait is
called a polygene
Beyond Mendel
Polygenic Inheritance
 example: corn length
• an ear of corn is controlled by two genes, A and B
• each dominant allele contributes to length, recessive
alleles do not contribute
AABB is the genotype with the largest length
phenotype
aabb is a genotype with the smallest length phenotype
Beyond Mendel
Polygenic Inheritance
 a true-breeding lines for longest and shortest ear lengths
are crossed
P generation AABB x aabb
F1 generation AaBb
 with four genes you start to see a range of lengths.
• continuous phenotypic traits