Transcript Chromosomal Basis of Inheritance

Chromosomal Basis of
Inheritance
Chapter 15
Objectives
• Be familiar with patterns of inheritance for
autosomal and sex linked genes
• Understand the concept of “Linked Genes”
• Understand how traits affected by incomplete
dominance and codominance differ from
autosomal dominant and autosomal recessive traits
• Understand how nondisjunction of chromosomes
can lead to disorders.
Linked Genes
• Linked genes are those that reside on the
same chromosome and tend to be inherited
together
• Humans have 23 pairs of chromosomes
– Autosomal genes reside on the autosomal
chromosomes (pairs 1-22)
– Sex-linked genes are found on the sex
chromosomes (pair 23, usually on the X)
Karyotype: picture of a complete set of chromosomes
Autosomal Genes
• Autosomal genes are
usually represented by
a pair of alleles
• The phenotype of the
gene reflects the
dominant/recessive
relationship of the
alleles
Autosomal Disorders
• Most autosomal genetic diseases are
autosomal recessive meaning the individual
need to be homozygous recessive to exhibit
the condition (example: cystic fibrosis)
• Huntington’s disease is an autosomal
dominant disorder meaning that is a single
Huntingtons allele is inherited, the
individual will have the disease.
Incomplete and Codominance
• Some alleles do not show a dominance
hierarchy
• Incomplete dominance: the phenotype of a
heterozygous genotype is intermediate in
appearance
• Codominance: each allele in the genotype
for a particular gene will be expressed in the
phenotype
Incomplete Dominance
(heterozygous genotype is intermediate in appearance)
Codominance
(each genotypic allele will be expressed)
Sex Linked Inheritance
• Males and females
differ in their sex
chromosome
combination (females
XX; males XY)
• Because the X contains
genes and the Y “does
not”, inheritance
patterns of sex-linked
genes vary between the
sexes
– recessive traits more
prevalent in males
Is the Disorder an Autosomal Recessive
Disorder or Is It Sex Linked?
Is This Disorder Autosomal
Recessive or Sex Linked?
Errors In Chromosomal
Inheritance
• Genetic disorders can
also occur due to
errors in the number of
inherited
chromosomes
• This condition arises
through a problem that
occurs during meiosis
Nondisjunction
• Failure of a pair of
chromosomes to separate
during gamete formation
• Genetic disorders like:
–
–
–
–
–
Down’s syndrome (#21, 3n)
Turner syndrome (XO)
Metafemale (XXX)
Kleinfelter Male (XXY)
Jacob’s Male (XYY)
X inactivation in female
mammals
• Although female mammals, including
humans, inherit two X chromosomes, one X
chromosome in each cell becomes almost
completely inactivated during embryonic
development.
– Barr body
Example of Barr Body
http://griffing.tamu.ed
u/BIOETCSITE/Biolo
gy01/Module11/Unit8
/Barr.htm
Sainato
AP Biology
Corona High School
• Nondisjunction
– Leads to aneuploidy:
• Aneuploidy is the condition of having less than or
more than the normal diploid number of
chromosomes, and is the most frequently
observed type of cytogenetic abnormality.
Abnormal Chromosome #
• Nondisjunction
– Leads to aneuploidy:
• Aneuploidy is the condition of having less than or
more than the normal diploid number of
chromosomes, and is the most frequently
observed type of cytogenetic abnormality.
Sainato
AP Biology
Corona High School
Alterations of chromosome
structure
The Key Points in Review
• Mendelian inheritance has its physical basis
in the behavior of chromosomes during
sexual life cycles.
• Morgan traced a gene to a specific
chromosome.
Key Points in Review
• Linked genes tend to be inherited together
because they are located on the same
chromosome.
• Independent assortment of chromosomes
and crossing over produce genetic variation
(recombinants)
Key Points in Review
• Geneticists can use recombination data to
map a chromosomes genetic loci.
• Chromosomal basis for sex is dependent
upon the organism.
Key Points for Review
• Sex-linked genes have unique patterns of
inheritance.
• Alterations of chromosome numbers or
structure cause some genetic disorders.