Human Heredity

Human Chromosomes

Humans have 46 total chromosomes

Two Categories– Autosomes- first 22 pairs– Sex Chromosomes- 23rd pair, determine

sex (gender)• X and Y• Males- XY• Females- XX

Punnett Square of Gender

Karyotype

Picture of chromosomes paired up with it’s homologous chromosome

– Commonly used for genetic testing

Taken during mitosis when chromosomes are visible

Dye added to show banding of genes

Karyotype Activity

Utah Website

Types of Disorders

Major Categories

– Chromosomal– Single Allele (single

gene)

Chromosomal Disorders- gaining or losing a piece/entire chromosome

– Gaining= 47 chromosomes

– Losing= 45 chromosomes

Why Chromosomal Disorders Occur

Non-disjunction- error in meiosis in which chromosomes fail to separate

– Monosomy- only getting one copy of a chromosome

– Trisomy- getting three copies of a chromosome

Examples

Down Syndrome- trisomy 21– Having 3 copies of

chromosome number 21

Turner’s Syndrome- monosomy X, girl only having one X chromosome

Single-Allele Traits

Disorder caused by a single gene

Types– Autosomal Dominant– Autosomal Recessive– Sex-Linked

Autosomal Dominant- if present will be expressed– Only need one bad

copy to be expressed

Autosomal Recessive- can be masked– Must have two bad

copies to be expressed

– Can skip generations

Carrier- a person who is healthy but has a faulty gene– Heterozygous

– Has no symptoms, but can pass on to children

Sex-linked

Disorder found on X or Y chromosome

– Found more in men because only have 1 X chromosome

• Men need just one bad copy to be expressed

Pedigree

Chart used to determine inheritance patterns within a family– Family genetic tree

– Males-squares– Females- circles

Autosomal Dominant

Autosomal Recessive

Sex-Linked

Pattern of Inheritance- patterns seen in a pedigree

– A. Dominant- in every generation– A. Recessive- can skip generations– Sex-linked- found more in males

Other Patterns of Inheritance

Sex-Influenced- having the same genotype but different expressions of that gene– Presence of hormones alters expression

Example: Pattern Baldness

In Males– NN- Normal hair– Nn- baldness– nn- baldness

In Females– NN- Normal hair– Nn- normal hair– nn- baldness

The heterozygous is different between males and females

X Chromosome Inactivation

Females turn “off” one of their X chromosomes

– Why? To compensate for males only having one X

– Barr body- name of dense region in nucleus created by turned off X

What Causes Disorders?

Simple Answer- Mutations

Germ-cell Mutation- change in DNA of a gamete cell

Somatic cell Mutation- change in DNA of a body cell

Chromosomal Mutations

Changes in the structure of a chromosome or loss/gain of a chromosome

Types– Deletion- loss of a piece– Duplication- repeating a piece– Inversion- piece breaks off, reverses, then

reattaches

– Translocation- chromosome piece breaks off, then attaches to another chromosome

– Non-disjunction- failure of chromosomes to separate during meiosis

Gene Mutations

Changes to nucleotides of DNA

Types– Substitution- exchanging one base for

another– Frame Shift- causes changes to the

reading of codons• Addition- adding a base• Deletion- removing a base

Remember DNA sequence is read three bases at a time (codon)

THE CAT ATE THE RAT- normal

THE BAT ATE THE RAT- substitution

THE CAT ATE THE RAT- normal

THE CCA TAT ETH ERA T- addition

THE ATA TET HER AT- deletion

End Result

Frameshift Mutations will make a wrong protein or one that is deficient

Substitution- can result in no change or wrong amino acid

Genetic Disorders

See handout sheet

– Write down how they are inherited and a general description

Linked Genes

Each chromosome is actually a group of linked genes– During meiosis the

chromosomes separate independently, not genes

Linked Forever?

No, crossing over separates and exchanges information

– Further apart of chromosome= more likely to be exchanged

Gene Map

Shows the relative locations of known genes for a chromosome

Detecting Human Disorders

Genetic Markers- short section of DNA that is related to a particular gene

Genetic Screening- examination of a person’s genetic make-up– Using- karyotypes, pedigrees, and genetic

markers

Genetic Counselor- medical guidance that informs a couple about problems that can affect offspring

– Amniocentesis- test used to withdraw fluid while baby is still in womb to test for genetic disorders

Human Genome Project- worldwide effort to determine the entire nucleotide sequence for the entire human genome

– Determined humans to have about 35,000 genes

– Created gene maps for each chromosome

Gene Therapy

Treatments for genetic disorders in which the faulty gene is replaced with a normal gene

Ethical Issues

Gene Therapy

Cloning- producing a new individual from one cell– Has exact same genetic make-up

Genetic Screening- how will information be used?