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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CONCEPT: CHROMOSOMAL THEORY OF INHERITANCE
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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CONCEPT: CHROMOSOMAL THEORY OF INHERITANCE
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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CONCEPT: CHROMOSOMAL THEORY OF INHERITANCE
● 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
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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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