Chapter 12

Patterns of Heredity & Human Inheritance

12.1: Mendelian Inheritance ofHuman Traits Pedigree- graphic representationof genetic inheritanceDifferent symbols are used torepresent: Males and females Individuals affected or unaffected by a particular

trait Carrier- a heterozygote for a recessive trait that

does have exhibit the trait in their phenotype, but “carries” the allele for the trait in his or her genotype

Recessive Disorders

Disorders that appear or are expressed when the genotype is homozygous recessive

Examples of recessive disorders:

1. Cystic fibrosis Results in accumulation of thick mucus in

lungs and digestive tract2. Tay-Sachs disease Lipids are not broken down in brain cells As lipids accumulate, brain function

declines and eventually leads to death Especially common on those of eastern-

European Jewish descent

3. Phenylkentonuria (PKU)Amino acid phenylalanine cannot be

broken downAccumulation leads to mental retardationTreated by a diet low in phenylalanine

Dominant Disorders

Disorders that appear or are expressed when the genotype is heterozygous or homozygous dominant

Examples (not necessarily disorders)

1. Cleft chin, widow’s peak, hitchhiker’s thumb, mid-digit hair

2. Huntington’s disease Results in breakdown of certain areas of

the brain No treatment

12.2: When heredity follows different rules

Most inheritance does not follow the simple patterns established by Mendel

Incomplete Dominance

Heterozygotes have an appearance that is in between the phenotypes of the two homozygotes

Capital letters will be used for both traits because neither is dominant

Flower color

Example problem for incomplete dominance:

Multiple Alleles

Every trait we have discussed so far has two alleles

Some traits have more than two possible alleles

However, each individual only has two of them (one on each of two homologous chromosomes)

A, B, O blood groups

Letters refer to carbohydrates found on the surface of red blood cells

Possible alleles for blood type:IA- carbohydrate A is presentIB- carbohydrate B is presenti- no carbohydrate is present

Both IA and IB are dominant over i

Possible Genotypes and Phenotypes

IAIA- type AIBIB- type BIAIB- type ABIAi- type AIBi- type Bii- type O

Example problem for multiple alleles:

Codominance

Notice that when IA and IB are paired together, both alleles are expressed, blood type is AB

Codominance- both alleles are expressed in heterozygotes, both alleles are dominant

Sickle Cell Disease- an example of codominance

Symptoms: Breakdown of red blood cells Clumping of cells & clogging of vessels Accumulation of sickle cells in spleen Heart failure, pain, fever, brain damage,

weakness, kidney failure

Genotypes & Phenotypes

N- normal S- sickle cells

SS- suffer from full-blown disease

SN- usually healthy, but may have some symptoms at high altitudes

NN- no symptoms

About 1 in 10 African Americans is a carrier for sickle cell disease, but the disease is very rare in Americans of other ancestry

Why is this gene so common if the effects are so serious?

Heterozygote advantage- heterozygotes for a particular trait have some advantage that homozygotes do not

In the case of sickle cell heterozygotes, they are resistant to malaria

In locations where malaria is prevalent, heterozygotes tend to live longer and produce more offspring

Malaria

Example problem for sickle cell disease:

Sex Determination

There are two possible sex chromosomes in humans, X or Y

Females- XXMales- XY

Sperm determines the sex of the offspring

Example problem for sex determination:

Sex-Linked Traits

Genes located on the sex chromosomesMost sex-linked genes are located on the

X chromosome- phenotype can be seen in both males and females

A few are located on the Y chromosome- phenotype can only be seen in males

Sex-linked disorders

Color-deficiency (blindness)Malfunction of light-sensitive cells in eyesRed-green- see gray tones instead of red

or greenInvolves several X-linked genesNormal color vision- see over 150 colorsColor-deficient- see fewer than 25 colors

Can females be colorblind?

Possible alleles:XB- normal Xb- color deficient

Possible genotypes & phenotypes: XBXB- normal female XBY- normal male XBXb- carrier female XbXb- color deficient female XbY- color deficient male

Example problem for sex-linked traits:

Can males be carriers of X-linked traits?

Why not?

HemophiliaX-linked geneExcessive bleeding when

injured- lack clotting factorMay bleed to death after

minor injuryExcess blood pools in joints

Duchenne muscular dystrophyProgressive weakening & loss of muscle

tissueDeath usually occurs by age 20

Polygenic Inheritance

Polygenic inheritance- the additive effects of two or more genes appear in a single phenotypic characteristic

These characteristics tend to occur on a continuum

What are some examples of human characteristics that occur on a continuum?

Skin tone

External Influences on Phenotype

External environment can affect the expression of certain genotypes

Examples of external influences:TemperatureLightNutritionChemicals

Internal Influences on Phenotype

Gene expression may be affected by internal environments:

Presence or absence of hormonesStructural differencesAge

12.3: Complex Inheritance of Human Traits

See examples from section 12.2 notes

Karyotype

Orderly display of magnified images of an individual's chromosomes

Can detect chromosomal abnormalities

Abnormalities in Chromosomal Number

Usually, a human embryo with an abnormal number of chromosomes is miscarried.

Some abnormalities in chromosomal number upset genetic balance less drastically

Causes of chromosomal number abnormalities

Nondisjunction- when members of a chromosome pair fail to separate

During Meiosis I: Homologous chromosomes do not separate All gametes have abnormal numbers

During Meiosis II: Sister chromatids do not separate 2 gametes are normal, 2 are abnormal

If either of these events occurs, surviving offspring that results from these abnormal gametes will have abnormal karyotypes

Disorders Resulting from Nondisjunction

Trisomy 21Result of nondisjunction in chromosome

21Affected individuals have 3 copies of

chromosome 21Leads to Down Syndrome

Down Syndrome

Klinefelter’s SyndromeExtra X in males- XXYSterilityBreast enlargementNormal intelligenceAlso includes multiple disjunctions- XXYY,

XXXY, XXXXY- more mental retardation

Klinefelter’s Syndrome

Jacob’s SyndromeExtra Y in males- XYYNormal malesTaller than average

Superwoman Syndrome (metafemales)Extra X in females- XXXNormal femalesLimited fertility

Turner SyndromeFemales lacking an X- XOShort statureWebbing of neckSterile, poor development of secondary

sex characteristicsNormal intelligence

Turner Syndrome