Complex Inheritanceand Human Heredity 11

The Big Idea

Human inheritance does not always follow Mendel’s laws.

Main Idea #1The inheritance of a trait over several

generations can be shown in a pedigree.

Recessive Genetic Disorders

• Remember - recessive traits can be masked by a dominant trait.

• Therefore, a person who is heterozygous for a characteristic may be considered a carrier for that trait since it will not be expressed.

• Even though a carrier is not affected by the trait, they can pass the trait on to future generations.

• In order for the recessive trait to show in the individual, the person must be homozygous recessive.

Common Recessive Disorders

• Cystic Fibrosis

• Affects the mucus-producing glands, digestive enzymes, and sweat glands

• Chloride ions are not absorbed into the cells of a person with cystic fibrosis but are excreted in the sweat

• Without sufficient chloride ions in the cells, a thick mucus is secreted

• The mucus clogs ducts in the pancreas, causes problems with digestion, and blocks some of the tiny passages in the respiratory system.

Common Recessive Disorders

• Albinism

• Caused by altered genes, resulting in the absence of melanin (color pigment) in the hair and eyes

• Common characteristics include: white hair, very pale skin, and pink pupils

• Tay-Sachs Disease

• Caused by the absence of an enzyme responsible for breaking down fatty acids

• The fats begin accumulating in the brain, destroying brain cells and mental deterioration.

Dominant Genetic Disorders• Some genetic disorders are controlled by a dominant

allele.

• People who have only one copy of this allele will display the trait. Only those who are homozygous recessive will not have the disorder.

• Huntington’s Disease

• Disease that gradually destroys the nervous system. Symptoms usually begin between the ages of 30 and 50.

• Achondroplasia is a genetic condition that causes small body size and limbs that are comparatively short. This is the most common form of dwarfism.

Pedigrees• A pedigree is a diagram that traces the inheritance of a

particular trait through several generations.

• Males are represented by squares, and females by circles.

• One who is affected by the trait will have a symbol that is colored in, while those that are unaffected by the trait will have an unfilled symbol.

Main Idea #2Complex inheritance of traits does not follow

inheritance patterns described by Mendel.

Complex Patterns of Inheritance

• Incomplete Dominance

• The heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes.

Complex Patterns of Inheritance

• Codominance

• Both alleles are expressed in the heterozygous condition.

Complex Patterns of Inheritance

• Another example of codominance is sickle-cell disease.

• Changes in hemoglobin cause blood cells to change to a sickle shape.

• People who are heterozygous for the trait have both normal and sickle-shaped cells.

Complex Patterns of Inheritance

• Multiple Alleles

• Some traits are controlled by more than 2 alleles for a particular gene.

• Human blood groups are controlled by 3 alleles - IA, IB, and i. IA and IB are codominant to each other, and dominant to i.

• There are 4 possible blood types - A (IAIA or IAi), B (IBIB or IBi), AB (IAIB), and O (ii).

Complex Patterns of Inheritance

• Sex Determination

• Sex chromosomes determine an individual’s gender.

• The X chromosome is much larger than the Y chromosome.

• Therefore, it makes sense to assume that X chromosomes carry more genes and genetic information than the Y chromosomes.

Sex-Linkage

• The presence of a gene on a sex chromosome is called sex linkage.

• Genes found on the X chromosome are called X-linked, and genes found on the Y chromosome are called Y-linked.

Example of Sex-Linked Trait

• Colorblindness is an X-linked trait, carried by sex chromosomes.

• The allele for colorblindness is represented by XC, while the normal allele is X.

• A female must have two XC alleles in order to be colorblind, but if a male carries the XC allele, he will definitely be colorblind.

Colorblindness Test

In each of the circles, see if you can identify a

hidden number. A person with normal vision will see the

numbers, while a person who is colorblind will see only the circle.

Another Example of Sex-Linkage

• Hemophilia is another example of a sex-linked trait.

• This rare disease causes blood to not clot properly.

• Failure of blood to clot may result in death.

• This trait is located on the X chromosome.

• Males are more likely to display this disorder since they only have to have one copy of the allele.

Polygenic Traits• Polygenic traits arise from the interaction of

multiple pairs of genes.

• Examples include skin color, height, and body mass.

Environmental Factors

• Sometimes, the environment affects how a trait is expressed.

• These influences include:

• environmental factors

• diet and exercise

• sunlight and water

• temperature

Twin Studies

• Twin studies can help scientists determine if a characteristic is caused by genetics or by environmental conditions (Nature vs. Nurture).

• Traits that appear frequently in identical twins are at least partially controlled by heredity.

• Traits expressed differently in identical twins are strongly influenced by environment.

Main Idea #3Chromosomes can be studied using

karyotypes.

Karyotypes

• A karyotype is a picture of a person’s chromosomes.

• Chromosomes are taken from the stage of metaphase and stained.

• They are then placed in order of decreasing size. The last pair is the sex chromosome pair.

• Disorders caused by an incorrect number of chromosomes or malformed chromosomes are the easiest to identify using a karyotype.

Normal Karyotypes

Telomeres

• Telomere caps consist of DNA associated with proteins.

• They serve a protective function for the structure of the chromosome.

Nondisjunction

• Homologous chromosomes separate during the final steps of meiosis to ensure that each developing sperm or egg receives one copy of each chromosome.

• Nondisjunction is the failure of a pair of homologous chromosomes to separate properly.

• Resulting sex cells will either have an extra chromosome (trisomy) or a missing chromosome (monosomy).

Down Syndrome

• One of the most common disorders due to nondisjunction is Down syndrome, also known as trisomy 21.

• Down syndrome occurs when the person has a third copy of the 21st chromosome.

• Common features include: short stature, heart defects, distinctive facial features, and mental disability.

Disorders Due to Nondisjunction of Sex Chromosomes

Klinefelter Syndrome

(XXY)

Jacob’s Syndrome

(XYY)

Turner’s Syndrome

(XO)

Trisomy X Syndrome

(XXX)

Types of Mutations• Mutations can involve an entire chromosome or a single

DNA nucleotide, and they may take place in any cell.

• Germ-cell mutations occur in an organism’s gametes (germ cells).

• These do not affect that organism, but may be passed on to their offspring the germ cell becomes fertilized.

• Somatic mutations occur in an organism’s body cells and can affect the organism.

• These mutations cannot be passed on to offspring.

• Some examples include skin cancer and leukemia.

• Lethal mutations cause death, usually before birth.

Chromosome Mutations

• Deletion (1)- results in the loss of a piece of chromosome due to the breakage of that chromosome; genetic information will be lost

• Duplication (2)- results in the copying of a segment of the chromosome

• Inversion (3)- a segment of a chromosome breaks off and reattaches itself to the chromosome in a reversed order

Chromosome Mutations

• Insertion (1) - a segment of a chromosome breaks off and reattaches itself to another homologous chromosome

• Translocation (2) - segments of chromosomes break off and exchange places on different chromosomes

Chromosome Mutations

• Nondisjunction is also a type of chromosome mutation. It affects the total number of chromosomes instead of pieces of individual chromosomes.