Patterns of Heredity (non-Mendelian)

1. Patterns of Heredity and
Human Genetics

2. Simple Dominant Heredity
 This type of heredity is what Mendel
observed.
 It only takes one dominant allele for an
organism to show a dominant trait.
 For example, the genotypes RR and Rr would
show the same phenotype of ROUND seeds.

3. Simple dominant traits
 Tongue rolling
 Hapsburg lip (protruding lower lip)
 Free earlobes
 Hitchhiker’s thumb
 Almond shaped eyes
 Thick lips
 Presence of hair on middle knuckles.

4. Incomplete dominance
 Phenotype of the heterozygote is
intermediate between those of 2
homozygotes.
 Example: Homozygous red flower (RR) is
crossed with a homozygous white-flowered
plant (R’R’), all the offspring will have pink
flowers.
 Neither allele of the pair is completely
dominant.

5. Incomplete dominance
RR’ RR’
RR’ RR’
R R
R’
R’
RR RR’
RR’ R’R’
R R’
R
R’

6. Why does this happen?
 R allele codes for an enzyme that produces red
pigment
 R’ allele codes for a defective enzyme that makes no
pigment
 If the genotype is RR’ it only makes half the pigment
thus causing the phenotype to be pink.

7. Codominance occurs when both alleles for a gene are
expressed in a heterozygous offspring.
In Codominance, neither allele is dominant or recessive, nor
do the phenotypes appear to blend. Both alleles of a gene are
active and influence the phenotype.
Codominant genes are written as capital letters with a
different letter for each phenotype.
Cows demonstrate codominance in regards to hair
color.
R and W
RR = Red
WW = White
RW = Roan (both red and white)

8. Example: White Cow (WW) x Red Bull (RR)
Results:
Genotype: RW
Phenotype: Roan
R R
W
W
RW RW
RW RW
* Pay attention to the ‘ it does not matter if it is on the 1st
or 2nd
letter

10. Codominance in humans
 Sickle cell anemia
 Most common in Americans whose families
originated from Africa
 1 in 12 African Americans is heterozygous
for the disorder.
 An individual who is homozygous for the
sickle-cell allele, the oxygen-carrying
protein (hemoglobin) differs by one amino
acid from normal hemoglobin.

11. Sickle-cell anemia
 The defective hemoglobin forms crystal-like
structures that change the shape of red blood cells.
 They are shaped like a sickle (half moon)
 This shape causes slow blood flow, blocked small
vessels and tissue damage

12. Polygenic inheritance
 Traits such as skin color and height vary over
a wide range.
 These wide ranges occur because these
traits are governed by many different genes.
 Polygenic inheritance is the inheritance
pattern of a trait that is controlled by 2 or
more genes.

14. Multiple phenotypes from
multiple alleles
Traits controlled by more than two alleles in a
population have multiple alleles
Blood type is an example of a single gene that has multiple alleles in humans.
Alleles for blood type:
IA
IB
i

15. Multiple alleles in humans

16. Sex determination
 Humans have 23 pairs of chromosomes.
 22 of these pairs are autosomal (matching
homologous chromosomes)
 Homologous autosomes look exactly alike.
 The 23rd
pair differs in males and females.
 These are sex chromosomes.

19. Sex-linked inheritance
 Traits controlled by genes located on sex chromosomes
are called sex-linked.
 Alleles for sex-linked traits are written as subscripts of
the X or Y chromosome.
 X and Y chromosomes are not homologous, therefore
the Y chromosome has no corresponding allele on the X
chromosome and no subscript is used.
 Any allele on the x chromosome of a male will not be
masked by a corresponding allele on the Y chromosome!

20. Sex linked traits in humans
 Sex linked traits are inherited on the sex chromosomes
 Most are located on the X chromosome
 Males pass an X chromosome to their daughters and a Y
chromosome to their sons
 Females pass an X to both
 If a son receives an X chromosome with a recessive
allele from his mother, he will express the trait because
there is no chance of inheriting a dominant allele from
his father to mask the trait (X and Y are not homologous)

22. XX
Xc
YXY
XXc
X Xc
X
Y
Example: colorblindness

23.  Each male child whose mother is a carrier for
a defect has a 50% chance of inheriting the
defect
 Each female child whose mother is a carrier
for a defect has a 50% chance of becoming a
carrier

24.  Red-green color blindness
 Color blindness is caused by the inheritance of
either of 2 recessive alleles at 2 gene sites on
the X chromosome that affect the red and green
receptors in the cells of eyes
 Hemophilia
 Inability to clot blood
 X-linked disorder; affects 1 in every 10,000
males
 Only affects 1 in 100 million females

25.  Males inherit the allele on the X chromosome
from carrier mothers
 A single recessive allele will cause the disorder
in males
 Females need 2 recessive alleles to inherit
hemophilia
 Queen Victoria’s family is the most well-known
study for hemophilia

26. Sex-Influenced Traits
 The presence of male or female sex
hormones influences the expression of
certain human traits.
 Estrogen
 Testosterone
 With this type of trait, males and females
have different phenotypes even when they
have the same genotype.
 These genes are located on autosomes.

27. Example: Pattern Baldness
 B = dominant, hair loss
 B’ = normal, no hair loss
 BB male = hair loss
 BB female = hair loss
 BB’ male = hair loss
 BB’ female = NO hair loss
 The differences in gene expression are due to
higher levels of testosterone in men.