COMPLEX INHERITANCE AND HUMAN HEREDITY
*CH. 11.1 BASIC PATTERNS OF HUMAN INHERITANCE
*MAIN IDEA – THE INHERITANCE OF A TRAIT OVER SEVERAL GENERATIONS CAN BE SHOWN IN A PEDIGREE.
*QUESTION: If someone looks more like one parent than the other, did that person inherit more genes from that parent?
*RECESSIVE GENETIC DISORDERS
*Mendel’s work was ignored for 30 years until scientists began looking at heredity.
*Recessive traits are expressed when the individual is homozygous recessive for that trait.
*Individuals with at least one dominant trait will NOT express the recessive trait.
*An individual who is heterozygous for a recessive disorder is called a carrier.
*Examples of recessive genetic disorders are:
*Cystic fibrosis
*Albinism
*Galactosemia
*Tay-Sachs disease
*RECESSIVE GENETIC DISORDERS CONTINUED
*CYSTIC FIBROSIS
*One of the most common recessive genetic disorders among Caucasians is cystic fibrosis, which affects the mucus producing glands, digestive enzymes and sweat glands.
*People develop a thick mucus that clogs the ducts in the pancreas, interrupts digestion, and blocks the tiny respiratory pathways in the lungs.
*People with cystic fibrosis are at a higher risk of infection because of the mucus in their lungs
*Treatments are physical therapy, medication, special diets, and the use of replacement digestive enzymes.
*Genetic tests are available to determine if a person is a carrier.
*RECESSIVE GENETIC DISORDERS
*ALBINISM
*Albinism is caused by altered genes, resulting in the absence of the skin pigment melanin in hair and eyes.
*Albinism is also found in animals.
*People with albinism have white hair, very pale skin, and pink pupils.
*Lack of pigment in eyes causes vision problems.
*TAY-SACHS DISEASE
*Gene for Tay-Sachs disease (TSD) is located on chromosome 15.
*Disease is predominantly among Jews of eastern European descent.
*TSD is caused by the absence of the enzymes responsible for breaking down fatty acids, which build up in the brain, inflating brain nerve cells and causing mental deterioration.
*DOMINANT GENETIC DISORDERS
*Some genetic disorders are caused by the dominant allele. So if you do not have the disorder you are homozygous recessive for the trait.
*Some types of dominant genetic disorders include:
*Huntington’s disease affects the nervous system with systems first appearing between the ages of 30 and 50 years old.
*Systems include: loss of brain function, uncontrollable movements and emotional disturbances.
*Genetic tests are available to detect this dominant allele, but there are no preventive treatment or cure for this disease.
*DOMINANT GENETIC DISORDERS CONTINUED
*Achondroplasia (most common form of dwarfism) have a small body size and limbs that are short.
*Individuals with achondroplasia have a normal life expectancy and will reach a height of about 4 feet.
*75% of individuals with achondroplasia are born to parents of average size.
*Believed to be caused by new mutation or a genetic change.
*PEDIGREES
*Pedigree is a diagram that traces the inheritance of a particular trait through several generations of a family.
*Symbols are used to illustrate the inheritance of a trait.
*Males are represented by squares =
*Females are represented by circles =
*Person who expresses the trait being studied is represented by a filled in square or circle =
*Person who does not express the trait is represented by an unfilled square or circle. =
*Half-filled square or circle means the person is a carrier. =
*PEDIGREES – Continued
*Horizontal line between 2 symbols shows that the individuals are married
*Brackets show the offspring of the parents. Offspring are listed in descending birth order from left to right and are connected to each other and their parents.
*Pedigree uses a numbering system where Roman numerals represent generations and individuals are numbered by birth order using Arabic numbers.
*PEDIGREE
*INFERRING GENOTYPES
*Pedigrees are used to help figure out the genotypes
*Pedigrees are also used to help figure out dominant and recessive traits.
*Dominant traits are easier to recognize because they are expressed
*Recessive traits are only seen if the person is homozygous recessive for the trait
*Need to follow the recessive trait for several generations to figure out which parents & grandparents were the carriers of the recessive allele.
*SECTION 11.2 – COMPLEX PATTERNS OF INHERITANCE
*MAIN IDEA – Complex inheritance of traits does not follow inheritance patterns described by Mendel
*QUESTIONS: What possible eye colors are there?
*Do you think that eye color is inherited by a simple dominant/recessive manner?
*INCOMPLETE DOMINANCE
*REVIEW – dominant/recessive traits the dominant trait is expressed, even if the organism is heterozygous.
*Incomplete dominance is when a heterozygous organism shows a blending of the dominant and recessive trait.
*Ex: cross a red flower with white flower and the heterozygous flower will be pink.
*Written: C=color for the trait, CRR for red flowers & CW for the white flower. A heterozygous flower color is written CRCW
*INCOMPLETE DOMINANCE PUNNETT SQUARE
*CODOMINANCE
*In codominance both alleles are expressed in the heterozygous condition.
*EX: Homozygous black chicken & homozygous white chicken will produce heterozygous black and white chickens.
*SICKLE CELL DISEASE
*Sickle cell disease is a codominant inheritance
*Common in people of African descent and affects the red blood cells ability to carry oxygen
*Ex: if you are heterozygous for sickle cell disease your body produces both normal blood cells and sickle cells
*People living in malaria areas being heterozygous for sickle cell disease also have a higher resistance to malaria
*Allows the sickle trait to continue to be passed down.
*MULTIPLE ALLELES
*Some forms of inheritance are determined by more than 2 alleles. This is referred to as multiple alleles.
*EX: blood
*BLOOD GROUPS IN HUMANS
*The different types of blood that humans can have are: A, B, AB, and O
*A & B are dominant to O; but A & B together are codominant
*Blood type is written:
*A = IA
*B = IB
*AB = IAIB
*O = ii
*Rh factors are either + or - & written Rh+ or Rh- and + is dominant over -
*SEX DETERMINATION
*Each cell in your body, except gametes, contains 46 chromosomes or 23 pairs of chromosomes.
*One pair of chromosomes, the sex chromosomes, determines an individual’s gender.
*There are 2 types of sex chromosomes, X & Y.
*XX chromosomes = girls
*XY chromosomes = boys
*Males determine sex of the baby
*The other 22 pairs of chromosomes are called autosomes
*DOSAGE COMPENSATION
*Human females have 22 pairs of autosomes and 1 pair of X chromosomes.
*Human males have 22 pairs of autosomes and 1 X and 1 Y
*There are a lot of genes on the X chromosomes, but Y chromosomes only contain genes that pertain to male characteristics
*In female body cells one X chromosome is randomly chosen to be turned off or X-inactivation
*Turned off X chromosome is referred to as a Barr body
*SEX-LINKED TRAITS
*If a trait is located on the X chromosome it is called a sex-linked trait or X-linked.
*Males only have 1 X chromosome, so if that trait is dominant or recessive it is expressed.
*Females have 2 X chromosomes, so their traits follow the dominant recessive pattern.
*COLORBLINDNESS (Red/Green)
*Colorblindness is a recessive X-linked trait.
*Punnett squares for X-linked traits are written:
*EX: XB = normal vision; Xb = colorblind; Y chromosome
*X-Linked colorblind Punnett square
XB Y
XB
Xb
XBXB XBY
XBXb XbY
*HEMOPHILIA
*Hemophilia is a recessive sex-linked disorder. People with hemophilia have a delayed clotting of their blood.
*Hemophilia was passed through a lot of the royal families
*Men died more frequently and at an early age because of the absence of clotting factors
*20th century learned about the clotting factors & now it is given to people with hemophilia.
*POLYGENIC TRAITS
*Polygenic traits are traits that are controlled by multiple pairs of genes.
*EX: skin color, height, eye color, and fingerprint patterns
*When looking at the frequency of polygenic traits the results you see will be a bell shaped curve.
*ENVIRONMENTAL INFLUENCES
*Environment also has an effect on phenotypes.
*EX: you may inherit a gene that gives you the tendency to have heart disease.
*Environment factors such as diet and exercise can contribute to the occurrence and seriousness of the disease.
*Other environmental factors that can affect phenotype are:
*Sunlight
*Water
*Temperature
*Ch. 10.3 – CHROMOSOMES AND HUMAN HEREDITY
*KARYOTYPE STUDIES
*Karyotypes is a study used by scientists to study the whole chromosomes using images of chromosomes stained during metaphase (mitosis).
*Sister chromatids are arranged by looking at their length, centromere location, and the banding.
*Arranged by decreasing size of the sister chromatids.
*22 autosome chromosomes are arranged first
*Sex chromosomes X’s & Y’s are placed last
*Information that karyotypes can give us are:
*Sex of the individual
*Different genetic disorders
*KARYOTYPE
*TELOMERES
*At the ends of each chromosomes are protective caps called telomeres.
*Scientists believe that the telomeres might be involved in both aging and cancer
*NONDISJUNCTION
*Nondisjunction is when the sister chromatids fail to separate.
*If nondisjunction occurs either during Anaphase I or Anaphase II the gametes will not have the correct number of chromosomes.
*Results: gametes either have an extra chromosome or is missing a chromosome
*Trisomy is when you have a set of 3 chromosomes of one kind.
*In humans, altering the # of chromosomes is associated with serious human disorders or death.
*DOWN SYNDROME
*Down Syndrome occurs when there is an extra #21 chromosome (trisomy 21)
*Characteristics for Down Syndrome include:
*Distinctive facial features
*Short stature
*Heart defects
*Mental disability
*Frequency of Down Syndrome increases with the age of the mother.
*SEX CHROMOSOME
*Nondisjunction with the sex chromosomes can result in the following conditions:
*Turner’s syndrome = XO
*Klinefelter’s syndrome = XXY
*Death = OY
*Other abnormalities with the sex chromosomes include: XXX, XYY