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