Chapter 27 Human Genetics

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Chapter 27 - Human Genetics

• 271 The Role of Chromosomes
• A. Chromosome number

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Basic facts about chromosome number in living
things

• 1. Each human sperm/egg has 23 chromosomes.
• 2. Each body cell has 23 pairs (46) of
  chromosomes

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• 3. Different organisms have different numbers of
  chromosomes.

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B. A way to tell chromosome
number

• Amniocentesis is a way of looking at the
  chromosomes of a fetus
• Amniotic fluid is taken from the mother

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• It contains some cells of the fetus which are
  then cultured
• When the cells divide, the chromosomes become
  visible when viewed under the microscope

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• An enlarged photo is made of the cell, and the
  chromosomes are cut out
• A chart called a karyotype is made from matching
  pairs of the chromosomes which were cut out

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• A newer technique for getting the cells of the
  fetus is called chorionic villus sampling
• The doctor can take a small piece of the placenta
  which has the same chromosomes as the baby

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• The chromosomes can then be studied to see if
  they are normal or if there are genetic defects

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C. Sex - A Genetic Trait

• 1. The last pair of chromosomes, or the 23rd
  pair of chromosomes are called the sex
  chromosomes.
• 2. Males have an X and a Y
  chromosome

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• 3. Females have two X chromosomes
• 4. Autosomes do not determine a persons gender.
  They are body chromosomes.

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272 Human Traits

• A. Incomplete dominance
• Def Neither gene is totally dominant to the
  other
• The heterozygous individual is a blend

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Ex Snapdragons

• Let R red gene
• Let R white gene
• RR red
• RR pink
• RR white

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• If red (RR) snapdragons were crossed with white
  snapdragons (RR), the heterozygous (RR)
  offspring would be pink

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Red Blood Cells Codominance

• Let R normal round RBC gene
• Let R sickle cell gene
• RR all normal RBCs
• RR sickle cell trait (½ the RBCs are normal
  and ½ are sickle cells

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• People with sickle cell trait usually dont have
  major health problems, but may not be as
  physically active as people with all normal RBCs.
  Thats because sickle cells dont carry as much
  oxygen as normal RBCs.

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• RR sickle cell anemia (all the RBCs are
  sickle cells)
• People with sickle cell anemia have serious
  health problems. Sickle cells dont carry enough
  oxygen, damaging tissues and the
  irregularly-shaped cells can clog capillaries

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B. Blood Types Multiple genes and Codominance

• Codominance Each gene is equally expressed.
• The four blood types are
• A, B, AB, and O
• The genes are A, B, and O

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• A and B are dominant to O.
• A and B are not dominant to each other they are
  codominant.

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Genes on the X chromosome - Sex-linked
traits

• There are genes on the X chromosome that are not
  on the Y chromosome.
• Females have 2 X chromosomes, males have 1 X
  chromosome.

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• 1. If a gene is on the X chromosome, a female
  would have 2 genes on her 2 X chromosomes, but
  the male would have 1 gene on his 1 X chromosome.

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• 2. Colorblindness, hemophilia, and Duchenne
  Muscular Dystrophy are sex-linked recessive
  traits.
• Hemophilia is a disorder that keeps a persons
  blood from clotting.

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• XC normal color vision
• Xc colorblind
• XCXC or XCXc normal color vision female
• XcXc colorblind female
• XCY normal color vision male
• XcY colorblind male

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• In order for a female to be colorblind, she would
  have to have 2 genes for colorblindness
• A male will only need 1 gene for colorblindness
  to be colorblind.

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• It is more likely to have a colorblind male than
  a colorblind female.

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273 Genetic Disorders

• Some people have too many or too few chromosomes
  in each cell.
• When the egg or sperm (that created the person)
  were made during meiosis, one of the pairs of
  chromosomes may not have separated.

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• One sex cell will have one too many chromosomes
  and the other will have one too few chromosomes.

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X
0
X
X
Y
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• A Y0 male will die before it is born.
• An X0 female (Turners Syndrome) cant make sex
  cells
• An XXY male (Klinefelters Syndrome) cant make
  sex cells

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• An XXX female is not harmed from the extra
  chromosome.
• Down Syndrome is caused by an extra chromosome
  21, an autosome. Down Syndrome children learn
  more slowly and often have heart problems.

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PKU Phenylketonuria

• PKU is a genetic disorder in which an amino acid
  does not break down as it should. Too much
  phenylalanine can harm the brain. Newborns are
  tested for PKU. A special diet can prevent
  harmful effects.

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Genetic Counseling

• The use of genetics to predict and explain traits
  in children
• If a genetic disease is found in a family, a
  couple may want to know the likelihood of having
  a baby with the disease

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• A pedigree can be constructed to show how a trait
  is passed along in a family.
• You can also determine the probability of the
  genetic disorder occurring in a couples children.

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• During a pregnancy, there are tests which can
  determine if a fetus does have a particular
  genetic disease.

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Dyslexia

• Sometimes known as word blindness.
• People will see and write some letters or parts
  of words backward.
• They may have difficulty reading

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• Genetics is a factor in having this condition