This document discusses several patterns of heredity in humans including simple dominant and recessive traits, incomplete dominance where the heterozygote phenotype is intermediate, codominance where both alleles are expressed, polygenic traits determined by multiple genes, and sex-linked traits located on the X or Y chromosomes. It provides examples like tongue rolling, cow coat color, and sickle cell anemia to illustrate these genetics concepts.

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.