Chapter 10

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Chapter 10Genes and Chromosomes

• Charles Page High School
• Stephen L. Cotton

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Section 10-1 Chromosome Theory of Heredity

• OBJECTIVES
• Relate genes to chromosomes.

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Section 10-1 Chromosome Theory of Heredity

• OBJECTIVES
• Explain how gene linkage and crossing-over affect
  heredity.

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Section 10-1 Chromosome Theory of Heredity

• OBJECTIVES
• Describe the patterns of inheritance for
  sex-linked traits.

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Section 10-1 Chromosome Theory of Heredity

• Gregor Mendel is known as the founder of the
  science of genetics
• However, he failed to ask an important question
  Where in the cell are these factors that control
  heredity?
• Time was precious he was promoted to abbot of
  the monastery

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Section 10-1 Chromosome Theory of Heredity

• By the early 1900s, cell biologists had
  discovered most of the major structures in the
  cells
• It seemed logical that the nucleus was the place
  for the genes location it was large and
  centrally located, plus the nucleus does contain
  the chromosomes!

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Section 10-1 Chromosome Theory of Heredity

• Walter Sutton, a young graduate student at
  Columbia University, arrived at the answer for
  gene location The factors (genes) described by
  Mendel are located on chromosomes
• This is the Chromosome Theory of Heredity

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Section 10-1 Chromosome Theory of Heredity

• Summary
• 1. Each gene occupies a specific place on the
  chromosome
• 2. A gene may exist in several forms (or
  alleles)
• 3. Each chromosome contains one allele for each
  gene

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Section 10-1 Chromosome Theory of Heredity

• The fact that genes are located on chromosomes is
  important
• Genes on a chromosome are linked together
• In other words, linked genes do not undergo
  independent assortment
• This was discovered by Thomas Hunt Morgan

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Section 10-1 Chromosome Theory of Heredity

• Morgan studied the fruit fly-- Drosophila
  melanogaster, which can produce a new generation
  every 4 weeks- thus the characteristics can be
  observed relatively quickly
• Morgan crossed pure-bred flies that had gray
  bodies and normal wings with pure-bred flies with
  black bodies and small wings

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Section 10-1 Chromosome Theory of Heredity

• Gray (G) is dominant over black (g), and normal
  wings (W) dominant over small wings (w). Thus,
  all of the F1 flies should be gray with normal
  wings (GgWw)- this did happen
• However, when the F1 flies (GgWw) were crossed
  with black small-winged flies (ggww), unexpected
  results were obtained!

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Section 10-1 Chromosome Theory of Heredity

• Note the results in the Punnett square in Fig.
  10-4, page 207
• The results indicated that the gene for body
  color and wing size were somehow connected
  (linked), thus they could not assort independently

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Section 10-1 Chromosome Theory of Heredity

• As Morgan studied more and more, they found that
  genes fell into distinct linkage groups-
  packages of genes that always tended to be
  inherited together
• red hair and freckles, blonde hair with blue
  eyes, etc.

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Section 10-1 Chromosome Theory of Heredity

• The linkage groups, of course, were chromosomes
• There is one linkage group for every homologous
  pair of chromosomes
• Drosophila has four linkage groups, and four
  pairs of chromosomes

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Section 10-1 Chromosome Theory of Heredity

• Studies have shown that even though some genes
  are linked, they dont seems to always show this
  result, and are called recombinants
• Why is this? Alfred Sturtevant (an associate of
  Morgan) proposed that the linkages could be
  broken some of the time

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Section 10-1 Chromosome Theory of Heredity

• If two homologous chromosomes were close to each
  other, sections might cross, break, and reattach
• this was called crossing-over
• Fig. 10-6, page 208
• This is good, because it increases variety gene
  linkage decreases variety

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Section 10-1 Chromosome Theory of Heredity

• Remember if two genes are close, then
  crossing-over is rare however, if two genes are
  far apart, then crossing-over is more common
• This allows gene mapping- to map the positions of
  the genes on the chromosomes
• Fig. 10-7, page 209

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Section 10-1 Chromosome Theory of Heredity

• There is one exception to the rule that every
  chromosome has a corresponding homologous
  chromosome
• discovered by Nettie Stevens
• these are the sex chromosomes
• Fig. 10-8, page 209

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Section 10-1 Chromosome Theory of Heredity

• The sex chromosomes are a seemingly mismatched
  pair
• The other chromosomes (which are the same in
  males and females) are called autosomes
• Female fruit fly XX sex chromosomes
• Male fruit fly X and Y chromosomes

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Section 10-1 Chromosome Theory of Heredity

• So, how are the sexes of the offspring
  determined?
• Meiosis separates the sex chromosomes, just like
  any others
• From the female gametes have 1 X
• Male gametes can be X or Y
• Male is responsible! Fig. 10-9, p.210

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Section 10-1 Chromosome Theory of Heredity

• From the Punnett square, the chances of
  malefemale is 11
• In addition to determining the sex of an
  individual, sex chromosomes can carry genes that
  affect other traits
• these are called sex-linked
• Morgan discovered this link

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Section 10-1 Chromosome Theory of Heredity

• Morgan crossed a white-eyed male Drosophila with
  a group of red-eyed females
• R red, and r white
• The F1 all had red eyes as expected
• The F2 was expected to have 3/4 red1/4 white,
  and it did

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Section 10-1 Chromosome Theory of Heredity

• However, Morgan noticed that all of the
  white-eyed flies were male not a single
  white-eyed female at all.
• This was originally called sex-linked, because
  only one sex had it
• Today, we know the characteristic is carried on
  the X chromosome, and is sometimes called
  X-linked

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Section 10-1 Chromosome Theory of Heredity

• Note Fig. 10-10, page 210
• Morgan wondered if a white-eyed female could be
  produced?
• Yes, by crossing a white-eyed male (XrY) with a
  heterozygous red-eyed female (XRXr)
• Fig. 10-11, page 211

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Section 10-1 Chromosome Theory of Heredity

• Sex-linked genes are not only found in
  Drosophila, but also in humans, as we will see in
  Chapter 11
• genes for color vision
• genes for blood clotting
• Sex-linked characteristics tend to occur more
  often in males. Why?

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Section 10-2Mutations

• OBJECTIVES
• Describe different kinds of mutations.

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Section 10-2Mutations

• OBJECTIVES
• Explain how mutations can affect heredity.

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Section 10-2Mutations

• Mutations are mistakes in duplicating genetic
  information and transmitting it to the next
  generation
• From a Latin word meaning change
• Not all mutations are harmful many have either
  no effect, or a slight harmless change once in a
  while they are actually beneficial

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Section 10-2Mutations

• Mutations may occur in any cell.
• If they affect the reproductive cells (called
  germ cells), it is called a germ mutation
• Mutations that affect other areas of the body are
  called somatic mutations

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Section 10-2Mutations

• Because somatic mutations do not affect the
  reproductive cells, they are not inheritable
• Many cancers are somatic mutations
• Both types (germ mutations and somatic mutations)
  can occur at 2 levels- the chromosome level, and
  the gene level

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Section 10-2Mutations

• Chromosomal mutations-involve segments of
  chromosomes, or whole chromosomes, or even entire
  sets of chromosomes
• Gene mutations- involve individual genes
• Note Fig. 10-12, page 212

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Section 10-2Chromosomal Mutations

• Whenever a chromosomal mutation occurs, there is
  a change in the 1) number or 2) structure of the
  chromosomes
• 4 types involving the structure
• 1. Deletions 2. Duplications
• 3. Inversions 4. Translocations

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Section 10-2Chromosomal Mutations

• 1. A deletion involves the loss of part of a
  chromosome Fig. 10-12, p.212
• a b c d e f ? a _ c d e f
• 2. The opposite of a deletion is a duplication- a
  segment is repeated
• a b c d e f ? a b b c d e f

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Section 10-2Chromosomal Mutations

• 3. Inversion- when a part becomes oriented in the
  reverse of its usual direction
• a b c d e f ? a e d c b f

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Section 10-2Chromosomal Mutations

• 4. A translocation occurs when part of one
  chromosome breaks off and attaches to another,
  nonhomologous chromosome
• a b c d e f a b c k l
• g h I j k l g h I j d e f

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Section 10-2Chromosomal Mutations

• Chromosomal mutations that involve whole
  chromosomes or sets of chromosomes result from a
  process known as Nondisjunction- the failure of
  homologous chromosomes to separate during meiosis
• It literally means not coming apart

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Section 10-2Chromosomal Mutations

• Nondisjunction results in an extra copy of a
  chromosome in one cell, and a loss of that
  chromosome from the other
• Many human disorders are the result of
  nondisjunction, and will be discussed in Chapter
  11

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Section 10-2Chromosomal Mutations

• Nondisjunction can also involve more than one
  chromosome
• If all the homologous chromosomes fail to
  separate, the result can be a dramatic increase
  in chromosome number- producing triploid (3N), or
  even tetraploid (4N) organisms

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Section 10-2Chromosomal Mutations

• The condition in which an organism has extra sets
  of chromosomes is called polyploidy
• almost always fatal in animals
• however, in plants the polyploidy conditions
  results in often larger and hardier than normal

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Section 10-2Gene Mutations

• Mutations can occur in individual genes, and can
  seriously affect gene function
• Any chemical change that affects the DNA molecule
  has the potential to produce gene mutations

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Section 10-2Gene Mutations

• The smallest changes, known as point mutations,
  affect no more than a single nucleotide
• When the point mutation involves the insertion or
  deletion of a nucleotide, the situation may be
  very serious, and is called a frameshift mutation
• Fig. 10-13, page 213

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Section 10-3 Regulation of Gene Expression

• OBJECTIVES
• Discuss gene interactions that influence gene
  expression.

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Section 10-3 Regulation of Gene Expression

• Just like the individual cells do not function by
  themselves in isolation, individual genes do not
  function in isolation
• their functions are regulated and controlled,
  thereby enabling a complex genetic system to
  function smoothly

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Section 10-3 Regulation of Gene Expression

• It has become clear that the following are
  critically important
• a) interactions between different genes
• b) interactions between genes and their
  environment (more on this later!)

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Section 10-3 Regulation of Gene Expression

• Dominance is one way that genes interact with
  each other
• in many cases, the dominant allele codes for a
  polypeptide that works, whereas the recessive
  allele codes for a polypeptide that does not work

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Section 10-3 Regulation of Gene Expression

• In 1760, the German scientist Josef Kölreuter-
  crossed white carnations (rr) with red carnations
  (RR).
• The result pink flowers (Rr) had appeared, a
  characteristic intermediate between the two
  parents
• Had they just blended? NO!

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Section 10-3 Regulation of Gene Expression

• When two of these F1 hybrids were crossed, the
  parents phenotypes reappeared- 1/4 were red 1/2
  were pink 1/4 were white
• Thus, in carnations the R allele is incompletely
  dominant over the r allele

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Section 10-3 Regulation of Gene Expression

• In incomplete dominance, the active allele does
  not compensate for the inactive allele, and the
  heterozygous phenotype is somewhere in between
  the two homozygous phenotypes
• Codominance - this is a condition in which both
  alleles of a gene are expressed- Note Fig. 10-16,
  p.217

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Section 10-3 Regulation of Gene Expression

• In other words, both genes are active
• Codominant alleles are written as capital letters
  with either
• subscripts, such as B1 and B2
• superscripts, such as HR and HW
• Codominance is seen in many organisms

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Section 10-3 Regulation of Gene Expression

• Example Red hair (HR) is codominant with white
  hair (HW) in cattle. The genotype HR HW are
  roan, or pinkish brown, in color because their
  coats are a mixture of red and white hairs
• Another example is chickens with black or white
  feathers erminette

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Section 10-3 Regulation of Gene Expression

• Not all traits are produced by single genes- Many
  are produced by the interaction of many genes,
  and these are said to be polygenic
• For example, at least 3 enzymes (each produced by
  a different gene) is responsible for the
  reddish-brown pigment in the eyes of fruit flies

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Section 10-3 Regulation of Gene Expression

• Different combinations produce different eye
  colors
• More complicated traits, such as your nose shape,
  or color and markings of an animals coat, are
  the result of interactions between large numbers
  of genes