Chapter 12

Patterns of Heredity and Human Genetics

 Making a Pedigree

•      A pedigree is a graphic representation of the genetic inheritance of ONE trait

•      Symbols

–    Circle - female

–    Square - male

–    Shaded in - shows the trait

–    Half shaded in - is a carrier, heterozygous individual that does not show the trait

–    Each horizontal row is a generation - represented by roman numerals

–    Parents are connected horizontally

–    Children are connected to parents with a vertical line

 

  Simple Recessive Heredity

•      Most genetic disorders are recessive

•      Cystic Fibrosis

•      1/25 white Americans is a carrier

•      1/2500 white Americans inherits the disorder

•      Defective protein in the plasma membrane

–    Formation and accumulation of thick mucus in the lungs and digestive tract

 

 Simple Recessive Heredity

•      Tay-Sachs Disease

•      Results in the absence of an enzyme that normally breaks down a lipid produced and stored in the central nervous system(CNS)

–    The lipid accumulates

•      Common among Ashkanazic Jews - Eastern Europe

•      Blue stained areas are swollen  neurons

 Simple Recessive Heredity

•      Phenylketonuria (PKU)

•      Absence of an enzyme that converts the amino acid, phenyalanine, to tyrosine.

•      Phenylalanine accumulates and damages the CNS (in milk, diet foods)

–    Can also damage a heterozygous fetus with blood from homozygous recessive mother

•      Common in those with ancestry from Norway, Sweden, or Iceland

 

 Simple Dominant Heredity

•      Cleft chin, widow’s peak, unattached earlobes, hitchhiker’s thumb (back more than 30 degrees), almond-shaped eyes, thick lips, mid-digital hair

 Simple Dominant Heredity

•      Huntington’s Disease

•      Breakdown of certain areas of the brain

•      Usually, dominant disorders like this disappear, because it kills before the individual can reproduce

•      In this disease, onset happens between 30 and 50

 

 Incomplete Dominance

•      The phenotype of heterozygous individuals is intermediate between those of the two homozygotes

•      Snap Dragon

–    Red Flowered (RR) x White Flowered (R’R’)

–    All F₁ will be pink(RR’)

–    What will the F₂ generation look like?

 Codominance

•      The phenotypes of both homozygotes is expressed in heterozygous individuals

•      Chickens

–    Black-feathered (BB) x White-feathered (WW)

–    All F₁ will have both black feathers and white feathers (BW) 

–    What will F₂ look like? 

 Multiple Phenotypes from Multiple Alleles

•       For many traits, though you only have two alleles, many can exist in a population

–    The trait is said to have multiple alleles

•       Pigeons

–    Three alleles govern feather color

–    B^A is dominant - ash red feathers

–    B allele is dominant to b, but recesive to B^A - blue feathers

–    b is recessive to both - chocolate-colored feathers

 

 Sex Determination

•      There are 22 pairs of homologous chromosomes called autosomes

•      The 23rd pair of chromosomes are called the sex chromosomes

•      Male XY

•      Female XX

 Sex-Linked Traits

•      Traits controlled by genes located on sex chromosomes are called sex-linked traits

•      The alleles are written as superscripts of the X or Y chromosome

–    Alleles on Y chromosomes don’t have a corresponding allele on an X chromosome

–    In males, if they have a recessive allele on their X chromosomes, there isn’t another allele to cover it up

 Fruit Flies

•      Thomas Hunt Morgan (1910) experimented with fruit fly eye color

•      Red Eyed Female (X^RX^R) x White-Eyed Male (X^rY)

•      Assume two females and two males, what will the kids look like?

•      Females - all red (X^RX^r)

•      Males - all red (X^RY)

 Polygenic Inheritance

•       The inheritance pattern of a trait is controlled by two or more genes

–    Skin color, height, corn cob length

–    Genes may be on the same chromosome or different chromosomes

–    Each gene may have two or more alleles

•       Each allele represented by an uppercase letter contributes a small, but equal, portion to the trait being expressed

–    The result is that phenotypes show a continuous range of variability

 Polygenic Inheritance

•       Hypothetical Example

•       Stem length in a plant is controlled by 3 different genes: A, B, D. 

–    Each gene is on a different chromosome and has two alleles (A & a, B & b, D & d)

–    Each plant will have 6 alleles for stem length

–    Each tall allele contributes 2 cm (4 cm base minimum)

•       What would a plant with a genotype of  AaBbDd  look like?

•       A population will follow a normal curve.

 

 Environmental Influences

•      Genetics only determine potential

•      External Environmental Influences

–    Temperature, nutrition, light, chemicals and infectious agents can all influence gene expression

–    In Siamese cats and arctic foxes, temperature has an effect on coat color

–    Leaves can have different sizes, thicknesses, and shapes depending on the amount of sunlight they receive

 Environmental Influences

•      Internal environmental influences

•      Horn size in males and female mountain sheep is different due to differing internal environments

–    Also applies to baldness in humans and feather color in peacocks

•      Age can also affect gene expression, though this isn’t completely understood

 

Sickle-Cell Disease

•      An example of codominance in humans

•      Common in African Americans and Americans with ancestry near the Mediterranean Sea

•      Homozygous – hemoglobin differs from normal by 1 amino acid

–    Changes the shape of the red blood cells (rbc)

–    Slow blood flow, block small vessels, and result in tissue damage and pain

 Sickle-Cell Disease

•      Heterozygous – produce both normal and sickled hemoglobin (codominance)

–    Enough that they don’t have major health problems

–    Show sickle-cell related disorders when oxygen isn’t readily available

 Blood Type

•      There are three alleles for the gene, “I”

–    IA, IB, and i

•      I^A,I^A or I^A, i – blood type A

•      I^B, I^B or I^B, i – blood type B

•      I^A, I^B – codominance, blood type AB

•      ii – blood type O

•      There are different molecules that are produced on the surface of the rbc – represented by A and B

•      Your immune system fights against blood cells with different molecules

•      So who can donate blood to whom?

•      I^A, i x I^B, i – What will be produced?

 

 Rh  Factor

•       Separate gene from ABO blood type

•       Simple heredity - “ Rh +” is dominant over “ Rh -” 

•       If an antiserum agglutinates your red cells, you are “ Rh +”  If it doesn't, you are “ Rh -”

 Blood Type Frequencies

•      O+  38%

•      A+  34%

•      B+  9%

•      O-  7%

•      A-  6%

•      AB+  3%

•      B-  2%

•      AB-  1%

 Nondisjunction  in Humans

•      A karyotype, a chart of chromosome pairs during metaphase, is used to detect these disorders

 

 

 Down Syndrome

•      Trisomy 21

•      The only autosomal trisomy in which affected individuals survive to adulthood

–    Occurs about 1 in 700 births

–    A group of symptoms, including some degree of mental retardation, results from trisomy 21

Nondisjunction in Sex Chromosomes

•       XO – Turner’s Syndrome

•       XXY –  Klinefelter  Syndrome

•       XXX –  Trisomy  X Syndrome

•       XYY Syndrome

•       Most of these individuals have some degree of mental retardation and they cannot have kids.

 Sex-Linked Traits in Humans

•      Red-Green Color Blindness

•      Recessive allele on the X chromosome

•      How will a boy get it?  A girl? 

 Sex-Linked Traits in Humans

•      Hemophilia

•      Recessive disease that prevents the blood’s ability to clot

•      In males – 1 in 10,000

•      In females – 1 in 100,000,000

•      Why the difference?

 

 Skin & Eye Color

•      Both polygenic traits

•      What was polygenic inheritance?

•      How will this work for skin?

•      How will this work for eye color? 

 MiniLab  (p. 327 in book)

•       Human eye color, like skin color, is determined by polygenic inheritance. You can detect several shades of eye color, especially if you look closely at the iris with a magnifying glass. Often, the pigment is deposited so that light reflects from the eye, causing the iris to appear blue, green, gray, or hazel (brown-green). In actuality, the pigment may be yellowish or brown, but not blue.

•       Procedure:

–    1. Use a magnifying glass to observe the patterns and colors of pigments in the eyes of 5 classmates.

–    2. Use crayons to make drawings of the 5 irises.

–    3. Describe your observations.

•       Analysis

–    1. Observe  How many different pigments were you able to detect in each eye?

–    2. Critique  From your data, do you suspect that eye color might not be inherited by simple Mendelian rules? Explain.

–    3. Analyze  Suppose that two people have brown eyes. They have two children with brown eyes, one with blue eyes, and one with green eyes. What pattern might this suggest?

 

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