Part 6: The Chromosomal Basis of InheritanceChapter 15

Date: 12/1/16

AP Bio Vocabulary of the Day(Stems, Prefixes, and Suffixes…Oh my!)

echin- spiny, prickly

Example: echinoderm (a phylum of sea creatures many of whom have spiny skin)

Fabulous Fact

The brain of an adult human weighs around 3 pounds (1.5 kg). Although it makes up just 2% of the body's weight, it uses around 20% of its energy.

What you must know:How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance.

The unique pattern of inheritance in sex-linked genes.

How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders.

How genetic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian inheritance.

AP Biology Standards addressed: Big Idea 3

How are chromosomes involved in inheritance???(Hint: think about Mendel’s laws)

Chromosome theory of inheritance:

Genes have specific locations (loci) on chromosomes

Chromosomes segregate and assort independently

Chromosomes tagged to reveal a specific gene (yellow).

Thomas Hunt Morgan

Drosophila melanogaster – fruit flyFast breeding, only 4 prs. Chromosome, with 1 pair of sex chromosomes (XX/XY)

Sex-linked gene: located on X or Y chromosome

Red-eyes = wild-type; white-eyes = mutantSpecific gene carried on specific chromosome – white eye gene carried on X chromosome

Sex determination varies between animals

Sex-linked genes

•Sex-linked gene on X or Y

•Females (XX), male (XY)– Eggs = X, sperm = X or Y

•Fathers pass X-linked genes to daughters, but not sons

•Males express recessive trait on the single X (hemizygous)

•Females can be affected or carrier

Transmission of sex-linked recessive traits

Sex-linked disordersColorblindness

Duchenne muscular dystrophy

Hemophilia

Types of Chromosomal InheritanceV. X-inactivation in Females

In females one X in a cell is deactivated and condenses into a Barr body

Process occurs randomly in each embryonic cell

This results in a mosaic of two types of cells in the body

X-InactivationBarr body = inactive X chromosome; regulate gene

dosage in females during embryonic development

• Cats: allele for fur

color is on X

• Only female cats can

be tortoiseshell or

calico.

.. in human females affected by anhidrotic ectodermal dysplasia in which a mutant gene on one X chromosome results in patches of skin with no sweat glands.

X Chromosome Inactivation in Humans

Types of Chromosomal Inheritance

I. Linked Genes

Genes located near each other on the same chromosome are inherited together and considered “linked”- the frequency of being inherited together is greater than 50%

If genes are far enough apart on the same chromosome the probability of crossing over is high and they behave as unlinked genes

Human developmentY chromosome required for development of testes

Embryo gonads indifferent at 2 months

SRY gene: sex-determining region of Y

Codes for protein that regulates other genes

Genetic Recombination: production of offspring with new combo of genes from parents

If offspring look like parents parental types

If different from parents recombinants

• If results do not follow Mendel’s Law of Independent

Assortment, then the genes are probably linked

Linked genes: located on same chromosome and tend to be inherited together during cell division

Crossing over: explains why some linked genes get separated during meiosis

the further apart 2 genes on same chromosome, the higher the probability of crossing over and the higher the recombination frequency

Types of Chromosomal Inheritance

II. Chromosomal Disorders

1. Abnormal Chromosome Number

Results from nondisjunction in meiosis

At fertilization leads to aneuploidy (too many or too few chromosomes)

Examples: Down Syndrome, Kleinfelter Syndrome, Turner Syndrome

Karyotyping can detect nondisjunctions.

Down Syndrome = Trisomy 21

Nondisjunction: chromosomes fail to separate properly in Meiosis I or Meiosis II

Klinefelter Syndrome: 47XYY, 47XXY

Nondisjunction

Nondisjunction

Turner Syndrome = 45XO

Types of Chromosomal Inheritance

II. Chromosomal Disorders (cont.)

2. Alterations to Chromosome Structure

Result from chromosome breakage

Deletion- chromosome fragment lost

Insertion- chromosome fragment attached

Inversion- order reversed

Translocation- chromosome segment moved to a nonhomologous chromosome

Chromosomal Mutations

Chromosomal Mutations

Types of Chromosomal InheritanceIII. Genomic Imprinting

During gamete formation, the allele from one parent is silenced

The result is expression of only the other parent’s allele

Leads to variation in phenotype

Unique to mammals and flowering plants

Genomic Imprinting in Humans

The Curious Case of the Calico and Tortoiseshell…

Calico or Tortoiseshell??? Dense or Dilute???

Dilute Dense

Genetics in the Real World: The Trouble Twins

Ms. Van’s Twins!