Chapter 15 - The Chromosomal Basis of Inheritance Flashcards
What is the chromosome theory of inheritance?
Developed by Walter S. Sutton, Theodor Boveri; according to this theory, Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment
What is Mendelian inheritance based on?
Chromosome behavior;
- Chromosomes and genes are both present in pairs in diploid cells
- Homologous chromosomes separate and alleles segregate during meiosis
- Fertilization restores the paired condition for both chromosomes and genes
What is the wild type?
The phenotype for a character most commonly observed in natural populations
What are mutant phenotypes?
Traits that are alternatives to the wild type; due to alleles assumed to have originated as changes, or mutations
What is a sex linked gene?
A gene located on either sex chromosome
What are Y-linked genes?
Sex chromosomes located on the Y chromosome
- Most Y linked genes are related to sex determination
What are X-linked genes?
The human X chromosome contains approximately 1,100 genes
- X chromosomes have genes for many characters unrelated to sex
- X linked genes follow specific patterns of inheritance
- For a recessive X linked gene to be expressed, a female needs two copies of the allele (homozygous) and a male needs only one copy of the allele (hemizygous)
*X linked recessive disorders are much more common in males than in females, i.e. color blindness
What is hemizygous?
Used to describe males who have only one locus for the x-linked allele
What is a Barr body?
The inactive X in each cell of a female condenses into a compact object called a Barr body, which lies along the inside of the nuclear envelope; most of the genes of the X chromosome that forms the Barr body are not expressed; in the ovaries however, Barr body chromosomes are reactivated in the cells that give rise to eggs, such that following meiosis, every female gamete has an active X
What are linked genes?
Genes located near each other on the same chromosome tend to be inherited together in genetic crosses; such genes are said to be genetically linked
Each chromosome has hundreds of thousands of genes
What is genetic recombination?
The production of offspring with combinations of traits that differ from those found in either P generation parent
- Genes that are far apart on the same chromosome have a recombination frequency of about 50%
What are parental types?
Offspring with a phenotype matching one of the parental phenotypes
What are recombinant types?
- Recombinants; offspring with non-parental phenotypes (new combinations of traits)
- A 50% frequency of recombination is observed for any two genes in different chromosomes
What process must occasionally break the physical connection between specific alleles of genes on the same chromosome?
The crossing over of homologous chromosomes; accounts for the recombination of linked genes
- Morgan discovered that genes can be linked, but the linkage was incomplete because some recombinant phenotypes were observed
- Recombinant chromosomes bring alleles together in new combinations of gametes
- Random fertilization increases even further the number of variant combinations that can be produced
- This abundance of genetic variation is the raw material upon which natural selection works
What is a genetic map?
An ordered list of the genetic loci along a particular chromosome
What is a linkage map?
A genetic map based on recombination frequencies
What is a nondisjunction?
An occasional mishap in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separated during meiosis II; as a result, one gamete receives two of the same type of chromosome and another gamete receives no copy
What is aneuploidy?
Results from the fertilization of gametes in which nondisjunction occurred; offspring of this type have an abnormal number of a particular chromosome
What is a monosomic zygote?
Has only one copy of a particular chromosome
What is a trisomic zygote?
Has three copies of a particular chromosome
What is polyploidy?
A condition in which an organism has more than two complete sets of chromosomes
- Triploidy (3n) is three sets of chromosomes
- Tetraploidy (4n) is four sets of chromosomes
- Polyploidy is common in plants, but not in animals
- Polyploids are more normal in appearance than aneuploids
What are the four types of changes that occur because of the breakage of a chromosome?
- Deletion; occurs when a chromosomal fragment is lost
- Duplication; repeats a segment
- Inversion; reverses orientation of a segment within a chromosome
- Translocation; moves a segment from one location to another
What is Down Syndrome a result of?
An aneuploid condition that results from three copies of chromosome 21 (affects 1 out of ever 700 children born in the US)
What are the two normal exceptions to Mendelian genetics?
One exception involves genes located in the nucleus and the other exception involves genes located outside the nucleus
In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance
What is genomic imprinting?
For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits; genomic imprinting involves the silencing of certain genes depending on which parent passes them on
- It appears that imprinting is the result of methylation of cysteine nucleotides
- Genomic imprinting is thought to affect only a small fraction of mammalian genes
- Most imprinted genes are are critical for embryonic development
What type of genes are found in organelles in the cytoplasm?
Extracellular genes or cytoplasmic genes
- Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules
- Extracellular genes are inherited maternally because the zygotes cytoplasm comes from the egg
What are the characteristics that make Drosophila melanogaster (fruit flies) a convenient organism for genetic studies?
- Produce many offspring
- A generation can be bred every two weeks
- Have only four pairs of chromosomes
What contribution did Morgan bring to understanding Mendelian inheritance?
Morgan’s experiments provided convincing evidence that chromosomes are the location of Mendel’s heritable factors (genes)
- Chromosomes and genes are both present in pairs in diploid cells
- Homologous chromosomes separate and alleles segregate during meiosis
- Fertilization restores the paired condition for both chromosomes and genes
Explain how crossing over can unlink genes?
- Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed
- He proposed that some process must occasionally break the physical connection between genes on the same chromosome
- That mechanism was the crossing over of homologous chromosomes
How is sex genetically determined in humans?
Chromosomal system for determining sex in mammals is X-Y;
XX - female
XY - male
A gene on the Y chromosome called SRY (sex determining region on the Y) is responsible for development of the testes in an embryo
Explain why sex-linked diseases are more common in males.
For a recessive x-linked trait to be expressed, a female needs to copies of the allele while a male needs only one copy of the allele
Some disorders caused by the recessive alleles on the X chromosome in humans
- Color blindness (mostly X-linked)
- Duchenne muscular dystrophy
- Hemophilia
Describe the process of X inactivation in female mammals. Explain how this phenomenon produces the tortoiseshell coloration in cats.
In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development;
The inactive X condenses into a Barr Body
If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character
Explain how genetic maps are constructed for genes located far apart on a chromosome.
Genetic maps are constructed to predict a recombination frequency;
Genes that are far apart on the same chromosome can have a recombination frequency near 50%
