The chromosomal basis of inheritance

Question 1

The chromosome theory of inheritance
states:

Answer 1

Mendelian genes have specific loci (positions) on
chromosomes
Chromosomes undergo segregation and independent
assortment

Question 2

Recombination of Unlinked Genes: Independent
Assortment of Chromosome

Answer 2

Mendel observed that offspring often display combinations of traits that differ from those of either parent. This phenomenon is known as genetic recombination.
Offspring with a phenotype that matches one of the parental phenotypes are referred to as parental types.
Offspring that display non-parental phenotypes, which result from new combinations of traits not seen in either parent, are termed recombinant types or recombinants.
A 50% frequency of recombination is observed for any two genes located on different chromosomes. This is because genes located on different chromosomes assort independently during meiosis, leading to a random assortment of alleles into gametes and resulting in a 50% chance of recombination occurring between any two genes on different chromosomes.

Question 3

Recombination of Linked Genes: Crossing Over

Answer 3

Morgan discovered that genes can be linked,
but the linkage was incomplete, as evident
from recombinant phenotypes
* Morgan proposed that some process must
sometimes break the physical connection
between genes on the same chromosome
* That mechanism was the crossing over of
homologous chromosomes

Question 4

Mapping the Distance Between Genes Using
Recombination Data: Scientific Inquiry

Answer 4

Sturtevant predicted that the farther apart two
genes are, the higher the probability that a
crossover will occur between them and
therefore the higher the recombination
frequency

Question 5

Linked genes

Answer 5

Genes located on the same chromosome that tend to be inherited together

Question 6

Genetic Map

Answer 6

• Alfred Sturtevant, one of Morgan’s students,
  constructed a genetic map, an ordered list of
  the genetic loci along a particular chromosome

Question 7

A linkage map

Answer 7

genetic map of a chromosome based on recombination
frequencies

Question 8

map units

Answer 8

Distances between genes can be expressed as
map units; one map unit, or centimorgan,
represents a 1% recombination frequency
* Map units indicate relative distance and order,
not precise locations of genes

Question 9

Far apart genes

Answer 9

Genes that are far apart on the same
chromosome can have a recombination
frequency near 50%
* Such genes are physically linked, but
genetically unlinked, and behave as if found on
different chromosomes

Question 10

Cytogenetic maps

Answer 10

indicate the positions of genes with respect to chromosomal features

Question 11

Alterations of chromosome number or structure causes some genetic disorders

Answer 11

• Large-scale chromosomal alterations often
  lead to spontaneous abortions (miscarriages)
  or cause a variety of developmental disorders

Question 12

• In nondisjunction

Answer 12

pairs of homologous chromosomes do not separate normally during meiosis
* As a result, one gamete receives two of the
same type of chromosome, and another
gamete receives no copy

Question 13

Aneuploidy

Answer 13

-Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
-Offspring with this condition have an abnormal
number of a particular chromosome

Question 14

Monosomic zygote

Answer 14

has only one copy of a particular chromosome

Question 15

A trisomic zygote

Answer 15

has three copies of a particular chromosome

Question 16

Polyploidy

Answer 16

a condition in which an organism has more than two complete sets of chromosomes
– Triploidy (3n) is three sets of chromosomes
– Tetraploidy (4n) is four sets of chromosomes
* Polyploidy is common in plants (e.g. Bananas
3N; Wheat 6N), but not animals
* Polyploids are more normal in appearance
than aneuploids

Question 17

Alterations of Chromosome Structure

Answer 17

• Breakage of a chromosome can lead to four
  types of changes in chromosome structure:
  – Deletion removes a chromosomal segment
  – Duplication repeats a segment
  – Inversion reverses a segment within a
  chromosome
  – Translocation moves a segment from one
  chromosome to another

Question 18

Aneuploidy of Sex Chromosomes

Answer 18

Nondisjunction of sex chromosomes produces a variety of
aneuploid conditions

Question 19

Klinefelter syndrome

Answer 19

the result of an extra chromosome in a male, producing XXY individuals - sterile, often with some female sex traits , an extra y chromosome doesn’t seem to matter

Question 20

Monosomy X, called Turner syndrome

Answer 20

produces X0 females, who are sterile; it is the only known viable monosomy in humans

Question 21

• The syndrome cri du chat

Answer 21

results from a specific deletion in chromosome
5
* A child born with this syndrome is mentally
retarded and has a catlike cry; individuals
usually die in infancy or early childhood

Question 22

Genomic Imprinting

Answer 22

• For a few mammalian traits, the phenotype
  depends on which parent passed along the
  alleles for those traits
• Such variation in phenotype is called genomic
  imprinting
• Genomic imprinting involves the silencing of
  certain genes that are “stamped” with an
  imprint during gamete production
• It appears that imprinting is the result of the
  methylation (addition of –CH3
  ) of DNA
• Genomic imprinting is thought to affect only a
  small fraction of mammalian genes
• Most imprinted genes are critical for embryonic
  development

Question 23

Inheritance of Organelle Genes

Answer 23

Extranuclear genes (or cytoplasmic genes) are
genes found in organelles in the cytoplasm
* Mitochondria, chloroplasts, and other plant
plastids carry small circular DNA molecules
* Extranuclear genes are inherited maternally
because the zygote’s cytoplasm comes from
the egg
* The first evidence of extranuclear genes came
from studies on the inheritance of yellow or
white patches on leaves of an otherwise green
plant