KEY CONCEPT A combination of methods is used to study human genetics.

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KEY CONCEPT A combination of methods is used to
study human genetics.
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Human genetics follows the patterns seen in other
organisms.

• The basic principles of genetics are the same in
  all sexually reproducing organisms.
• Inheritance of many humantraits is complex.
• Single-gene traits areimportant in
  understandinghuman genetics.

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Females can carry sex-linked genetic disorders.

• Males (XY) express all of their sex linked genes.
• Expression of the disorder depends on which
  parent carries the allele and the sex of the
  child.

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A pedigree is a chart for tracing genes in a
family.

• Phenotypes are used to infer genotypes on a
  pedigree.
• Autosomal genes show different patterns on a
  pedigree than sex-linked genes.

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• If the phenotype is more common in males, the
  gene is likely sex-linked.

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• Chromosomes and Sex Determination
• Sex Chromosomes and Autosomes
• Sex chromosomes- discovered in 1905 by Nettie
  Stevens, female XX, male XY. X Y
  chromosomes are nonhomologous most of genes on
  one chromosome have no matching alleles on other
  chromosome. Many sex linked genes have nothing to
  do w/ sexual characteristics, IEgt eye color in
  Drasophila
• X-linked genes- sex-linked genes found on X
  chromosome
• Y-linked- sex-linked genes found on Y chromosome
• Karyotype- grouping by type size of chromosomes
  from a cell. There are 46 chromosomes in each
  human karyotype.
• Autosomes- all the chromosome pairs except the
  sex chromosomes

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• Sex Determination
• During meiosis sex chromosomes segregate with the
  result
• ½ males sperm carries X chromosome and ½ Y
  chromosome
• all eggs will have X chromosomes since females
  only have X chromosomes
• If sperm w/ X chromosome fertilizes offspring
  will be female
• If sperm w/ Y chromosome fertilizes offspring
  will be male
• 50/50 male vs. female
• Many organisms have no sex chromosomes
• Bees ants- sex is determined by total
  chromosome . Males are unfertilized and
  therefore haploid. Females are from fertilized
  eggs are diploid.
• Fish reptiles- some have no sex chromosomes
  instead sex is determined by environmental
  factors, IEgt alligators gender determined by
  temperature at which eggs develop

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• Sex-Limited Traits
• Sex-limited traits- autosomal traits that are
  expressed on only one sex.
• Carried by both male female
• Activated by hormone from one sex but not the
  other, IEgthuman males beard growth, human
  females milk production, male lion mane
• Embryos have autosomal genes that produce
  undifferentiated gonads undeveloped external
  sex organs. Genes on sex chromosomes cause these
  gonads to develop into either male or female
  genitalia, which in turn start making the
  hormones that cause male/female sexual structures
  characteristics
• Sex-influenced Traits
• Sex-influenced traits- are expressed in both
  sexes, but are expressed differently, IEgt
  baldness is dominant in the presence of male sex
  hormones, female sex hormones cause the gene to
  act recessive. A woman may become bald if she two
  genes for baldness.

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Several methods help map human chromosomes.

• A karyotype is a picture of all chromosomes in a
  cell.

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• Karyotypes can show changes in chromosomes.

• deletion of part of a chromosome or loss of a
  chromosome
• large changes in chromosomes
• extra chromosomes or duplication of part of a
  chromosome

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• Chromosome Mutations
• Chromosome mutations- changes in genetic material
  that involve entire chromosomes or pieces of
  chromosomes.
• They usually occur in meiosis or mitosis
• Can involve rearrangements of chromosome parts or
  changes in of chromosomes
• If occurs during gamete production the mutation
  can be passed on to offspring
• Some are harmful- some are helpful
• Changes in Chromosome Structure
• There are 4 types of chromosomes mutations
• Deletion- one or more genes are lost from a
  chromosome during division. This can occur if
  homologous chromosomes cross over unequally
  during meiosis.
• Duplication- a chromosome receives an extra piece
  in an unequal cross over which then duplicates
  some of the genes.
• Translocation- a whole or part of a chromosome
  attaches to a chromosome in a different pair. It
  may introduce too many or too few genes into the
  daughter cell. It may also break up important
  gene arrangements since some genes interact with
  other nearby genes.
• Inversion- a fragment of a chromosome breaks, off
  turn around, and reattaches in reverse order. It
  may cause no problem however it rearranges the
  gene sequence which can be a problem with genes
  that interact with another gene.

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• Changes in Chromosome Number
• Nondisjunction- the failure of chromatids or
  chromosomes to separate during cell division
  (mitosis or meiosis).
• Nondisjunction in mitosis- organism is unharmed
  but cell may die
• Nondisjunction in meiosis- results in abnormal
  gamete that can produce abnormal offspring
• Nondisjunction that occurs in meiosis leads to
  too many or too few chromosomes in an egg or
  sperm cell. Both monosomy trisomy will usually
  kill the embryo although sometimes the off spring
  survives with developmental difficulties.
• Monosomy- zygote receives only one of a
  particular type of chromosome instead of two like
  a normal zygote
• Trisomy- zygote receives three chromosomes of a
  particular type instead of two. Example of
  trisomies where the offspring survives with
  health problems include

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• Trisomy- zygote receives three chromosomes of a
  particular type instead of two. Example of
  trisomies where the offspring survives with
  health problems include
• Down syndrome- a type of trisomy of chromosome 21
  that causes severe mental retardation
• Leukemia- cancer of the bone marrow from trisomy
  on chromosome 21
• Alzheimers disease- degenerative brain disease
  that leads to dementia from trisomy on chromosome
  21
• Edwards Syndrome- trisomy on 18-The survival rate
  of Edwards Syndrome is very low. About half die
  in utero. Of liveborn infants, only 50 live to 2
  months, and only 510 will survive their first
  year of life. Major causes of death include apnea
  and heart abnormalities
• Klinefelters- XXY- male with small genitalia,
  female-like breasts, sterile
• Turner syndrome- monosomy X is the most common.
  Turner syndrome does not typically cause mental
  retardation or impair cognition. Women with
  Turner syndrome are almost universally infertile.

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• Polyploidy- Nondisjunction occurs in all the
  pairs of chromosomes at once. The organism has
  three or more sets of chromosomes. It can result
  from nondisjunction or from the polyploid gamete
  of one species unites with haploid gamete of
  another species.
• Animal with polyploidy- die
• Plants w/ polyploidy- can produce a more
  resilient plant. It is an important source of
  genetic mutation in plants, IEgt wheat, potatoes,
  oats.