Mendelian Inheritance of Human Traits

Simple Recessive Heredity

• When a person gets two recessive alleles.

• Simple Recessive Genetic Disorder

–Cystic Fibrosis (thick accumulation of mucus in lungs and digestive tract)

–Tay-Sachs (disease of the central nervous system)

–Phenylketonuria-PKU (results in damage to central nervous system)

Simple Dominant Heredity

• When a person gets at least one dominant allele.

• Simple Dominant Traits

–Tongue Rolling

–Huntingtons Disease (results in a breakdown of certain areas of the brain)

Complex Patterns of Inheritance

Incomplete Dominance

• Not all alleles are dominant and recessive like the ones Mendel studied in his pea plants. Some alleles are equally strong and neither are masked by the other.

• When both alleles are present, they are both expressed in the phenotype. The hybrid (heterozygote) is a blend of both alleles.

• An example of this is found in carnations. When purebred white carnations (WW) are crossed with purebred red carnations (RR), the result is a pink carnation (RW). When the heterozygote is produced, both alleles are represented by different capitalized letters.

Incomplete Dominance

• a heterozygote has a different phenotype as the two homozygotes

–Example-Red flowers are RR, white are WW, the heterozygous condition, RW produces pink flowers

Codominant

• Co-dominance, is a situation in which both alleles are equally strong and both alleles are visible in the heterozygous genotype.

• An example of co-dominance is found in chickens. When white chickens are crossed with black chickens, the result is not a grey chicken, but a chicken with both black and white feathers.

Codominant

• in a heterozygote individual both phenotypes are expressed equally

– Example – cross a black chicken (BB) and a white chicken (WW) the heterozygote wouldn’t be grey, the chicken has both black and white feathers.

codominance

EX.

–Example: Blood type gene has three different alleles (each person has two of the three possibilities)

• Type A allele is dominant and produces A type protein on red blood cells

• Type B allele is dominant and produces B type protein on red blood cells

• Type O allele is recessive (defective) allele and produces no protein on red blood cells

Codominant, cont.

• Blood types are controlled by multiple alleles. There are actually three different alleles; A, B, and O that determine a person's blood type. (Although there are three alleles possible, remember that each person only has two genes for every trait.)

• Of the three alleles, A and B show co-dominance. This means that a person possessing both A and B alleles as their genotype, has AB blood because both alleles are expressed in the phenotype. The O allele however, is recessive to both the A and B allele. Therefore, a person possessing both A and O alleles will have a blood type of A. Likewise, a person with both B and O alleles will be blood type B because B is dominant, and therefore, masks the O allele.

Blood Type

Genotype Phenotype

AA or AO A

BB or BO B

AB only AB

OO only O

Blood Type

Sex-linked Traits

• There are 22 pairs of chromosomes in your body that are called autosomes.

• The other pair are called sex chromosomes. They are called “X” and “Y”.

• Female genotype = XX

• Male genotype = XY

Sex-Linked Traits, cont.

• Some human traits are determined by genes carried on the sex chromosomes, most of these genes are on the X chromosome.

• Color blindness and hemophilia are examples of sex-linked traits.

Sex-Linked Traits

X chromosome

Y chromosome

Boy or Girl? The Y Chromosome “Decides”

What are Sex Linked Traits?

• In 1910, Thomas Morgan discovered traits linked to sex chromosomes in fruit flies.

• Some genes are attached to the X and Y chromosomes

• EXAMPLE: In humans, colorblindness and baldness are found on the X chromosomes

Punnett Square: What sex will the offspring be?

X Y

X X X X Y

X X X YX

50% chance of a male or a female child.

• - the inheritance pattern of a trait that is controlled by two or more genes.

–Examples in humans include

–hair color (AABBCC)

–eye color

–skin color

Polygenic Inheritance

Pedigree Charts

Pedigree Charts

A Pedigree chart is a graphic representation of genetic inheritance.

–kind of like a family tree

Pedigree Charts, cont.

• When you carefully make a pedigree chart you should be able to tell which individuals are possible carriers and the probability that future offspring will express the trait shown.