Chromosomes and
Inheritance
Chapter 12-1
Objectives
• Distinguish among inheritance patterns– Dominant, recessive, codominant, sex-linked,
polygenic, incomplete dominance, multiple alleles• Explain how mutations in the DNA sequence
can sometime result in change of an organism• Explain how mutations in gametes can result
in change in the offspring
Vocabulary
• Amniocentesis • Carrier • Chromosome
map• Deletion• Frame shift
mutation• Genetic disorder• Germ-cell
mutation
• Inversion• Lethal mutation• Monosomy • Nondisjunction• Pattern of
inheritance • pedigree• Point mutation• Polygenic trait• Sex-influenced
trait• Sex linkage• Single-allele trait• Somatic mutation• Substitution• Translocation• Trisomy
Sex Determination
• X is female and Y is male (smaller chromosome)
• After meiosis II, one cell gets X and one get Y (from male parent)
• 50% chance of being male or female
Sex Linkage
• Sex linkage is on sex chromosome• More genes on X chromosome than on Y• When on X, called X-linked genes• When on Y, called Y-linked genes
Linkage Groups
• Genes located on one chromosome and are inherited together
• Crossing over is the exchange of genes– Causes new gene combinations
Chromosome Mapping• The farther apart on the chromosome, the
likelier crossover will happen• Chromosome map shows the linear sequence
of genes on the chromosome through breeding experiments
• One map unit are 2 genes separated by crossing over 1% of the time (closer together, less likely to be seperated)
Mutation
• Change in DNA of an organism• Germ mutation- no affect, in gametes
(offspring?) • Somatic mutations- in body cells, do not affect
offspring• Lethal mutations- cause death usually before
birth• Some mutations are beneficial
Chromosome Mutations• Deletion is the loss of a piece of chromosome• Inversion is when a piece breaks off and
reattaches backwards• Translocation is when a breaks off and
reattaches to another nonhomologous chromosome
• Nondisjunction is when the chromosome doesn’t separate and the gamete gets an extra
Gene Mutations• Point mutation-substitution, addition, or
removal of a nucleotide• Substitution- one nucleotide is replaced with
another and makes a new codon– Sickle cell anemia- adenine is substituted for
thymine • Insertion is when a nucleotide is added and
deletion is when one is lost– Both are serious and cause frame shift mutation
(all codons moved)
Human Genetics
Chapter 12-2
Studying Human Inheritance
• Geneticists focus on disease • Usually study phenotype of members of the
same family and make a pedigree• Patterns of inheritance are predictable
patterns throughout generations• Carriers do not express allele but can pass it
on to offspring (recessive)
Pedigree Chart
Genetic Traits and Disorders
• Genetic disorders are diseases or debilitating conditions that have genetic basis
Traits Controlled by a Single Allele• Single allele traits controlled by a single allele of
a gene• Huntington’s Disease controlled by a dominant
allele – Most people do not know they have the disease until
after they have children (in their 30s or 40s)– Discovered genetic marker (short section of DNA that
is known to have association with nearby gene)• Others are controlled by homozygous recessive
traits– Cystic fibrosis and sickle-cell anemia
Traits Controlled by Multiple Alleles
• 3 or more alleles of the same gene for a single trait– ABO blood type (IA, IB, i)• A has A antigen, B has B antigen, AB has both, O has
none• A person with Rh- blood cannot receive Rh+ blood
Blood Typing
Polygenic Traits
• Controlled by 2 or more genes• Show many degrees of variation • Also influenced by environment (ex: height)
X-Linked Traits
• Found on the X chromosome• Colorblindness, Hemophilia, Duchenne
muscular dystrophy• Not all are diseases, mostly code for protein
Sex-Influenced Traits
• The presence of male or female sex hormones influence traits
• Ex: male pattern baldness– Both male and female homozygous will lose hair– Heterozygous males will lose hair, female will not
Disorders Due to Nondisjunction• Usually causes death• Monosomy -1, trisomy +1 chromosome– Down Syndrome (trisomy-21) has an extra
chromosome 21– Klinefelter’s Syndrome (XXY) has feminine
characteristics, mental retardation, infertile– Turner’s Syndrome (X) is female but does not
mature, infertile• Just Y does not survive
Detecting Human Genetic Disorders
• Genetic screening examines the genetic make-up through karyotype or blood tests (+ or – proteins)
• Genetic counseling is medical guidance for people at risk
• Can also test fetus– Amniocentesis removes fluid from around fetus– Chorionic villi sampling removes tissue from
between the uterus and placenta• Or immediately after birth– Some babies have phenylketonuria (PKU) which
prevents the digestion of phenylananine and can cause brain damage • Can be prevented by removing from the diet