ch.15 - chromosomal theory of...

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BIOLOGY - CLUTCH

CH.15 - CHROMOSOMAL THEORY OF INHERITANCE

CONCEPT: CHROMOSOMAL THEORY OF INHERITANCE

Chromosomal theory of inheritance: chromosomes are the carriers of genetic material.

● Independent Assortment – alleles for different characters sort independently of each other during gamete formation

● Law of Segregation – two alleles for a given character separate during gamete formation, and go to different gametes

● The chromosomal makeup of gametes depends on the arrangement of homologous chromosomes during metaphase I

EXAMPLE:

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Thomas Hunt Morgan conducted experiments with fruit flies (Drosophila melanogaster) that provided the first evidence for

inherited genes being located on chromosomes.

● Wild type – the phenotype for most organisms (what you might find “in the wild”)

□ Wild type flies have red eyes.

● Only the male flies in Morgan’s experiments showed the mutation for white eyes.

EXAMPLE:

● Sex-linked gene – gene located on a sex chromosome

□ The gene for eye color is linked to the sex of the fly.

● Sex chromosomes – chromosomes that contain genes that determine the sex of the organism, X/Y in humans

● There are few genes located on the Y chromosome, so there are few Y-linked genes.

● The X chromosome contains many genes, so there are many X-linked genes.

EXAMPLE:

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Women have two X chromosomes, but men have only one, so the expression of genes on the X chromosome must be

balanced between the sexes. This is referred to as dosage compensation.

EXAMPLE: ● X-inactivation – one X chromosome must be inactivated in women

□ Barr bodies – the highly condensed, inactive X chromosome

□ Which X chromosome will inactivate is randomly determined in each cell of the embryo during X-inactivation.

● Mosaicism – when an organism has cells with more than one genotype

EXAMPLE:

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● Linked genes – genes that are located near each other on a chromosome, and tend to be inherited together.

● Parental type – offspring with the same allele combinations seen in the parents

● Recombinant – offspring with different allele combinations than those of the parents

□ Genetic recombination is due to crossing over during prophase I of meiosis

EXAMPLE:

● Genetic map – an ordered list of gene loci along a chromosome

● Recombination frequency – the frequency with which a cross over between two genes on a chromosome will take place

□ The further apart two genes are the more likely a crossover will occur between them

□ Genes far enough apart behave as if unlinked, because cross over is nearly assured

● Linkage map – a genetic map based on recombination frequency

EXAMPLE:

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CONCEPT: CHROMOSOMAL ABNORMALITIES

Nondisjunction – when sister chromatids, or homologs fail to separate during cell division

● Aneuploidy – the wrong number of chromosomes, can be as a result of nondisjunction

□ Monosomy – one chromosome instead of the normal two

□ Trisomy – three chromosomes instead of the normal two

● Polyploidy – an organism with more than two complete sets of chromosomes

EXAMPLE:

● Deletion – loss of a chromosome fragment

● Duplication – can be cause by attachment of extra chromosome fragment

● Inversion – chromosome fragment attaches in reverse orientation

● Translocation – chromosome fragment attaches to a nonhomologous chromosome

EXAMPLE:

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CONCEPT: NON-MENDELIAN INHERITANCE

When Mendel performed his test crosses he saw that, in the characters studied, it didn’t matter which parent gave the allele.

● Genomic imprinting – when phenotype is determined by the parent that gave the allele

□ During gamete formation alleles for particular genes get silenced so that the offspring only expresses one allele.

□ Imprints are carried in the somatic cells of offspring, but removed from germ cells.

EXAMPLE:

● Mitochondria and chloroplasts contain their own DNA, and exhibit non-Mendelian inheritance.

□ Mitochondria are inherited from the mother.

EXAMPLE:

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ch.15 - chromosomal theory of...

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