Biology Test 4 - Ch 12-15 > Chapter 15 - Chromosomal Basis of Inheritance > Flashcards
Chapter 15 - Chromosomal Basis of Inheritance Flashcards
Aneu
= without
Cyto
= cell
Hemo
= blood
Mono
= one
Non
= not
Dis
= separate
Poly
= many
Re
= again
Com
= together
Bin
= two at a time
Trans
= across
Tri
= three
Soma
= body
Aneuploidy
A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.
Barr Body
A dense object lying along the inside of the nuclear envelope in female mammalian somatic cells, representing an inactivated X chromosome.
Crossing Over
The reciprocal exchange of genetic material between nonsister chromatids during Prophase I of meiosis.
Deletion
A deficiency in a chromosome resulting from the loss of a fragment through breakage.
Duplication
An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated.
Genetic Recombination
General term for the production of offspring that combine traits of the two parents.
Hemizygous
Having only a single copy of a gene instead of the customary two copies.
Inversion
An aberration in chromosome structure resulting from reattachment in a reverse orientation of a chromosomal fragment to the chromosome from which the fragment originated.
Linked Genes
genes located close enough together on a chromosome to be usually inherited together.
Monosomic
Referring to a cell that has only one copy of a particular chromosome, instead of the normal two.
Mosaic
A person or a tissue that contains two or more types of genetically different cells.
Nondisjunction
An error in meiosis, in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly.
Parental Type
An offspring with a phenotype that matches one of the parental phenotypes.
Polyploidy
A chromosomal alteration in which the organism possesses more than two complete chromosome sets.
Recombinant
An offspring whose phenotype differs from that of the parents; also called recombinant type.
Sex-linked gene
A gene located on a sex chromosome.
Translocation
An aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome.
Trisomic
Referring to a cell that has three copies of a particular chromosome, instead of the normal two.
Wild Type
An individual with the normal(most common) phenotype.
X-linked
A gene located on the X-chromosome.
Chromosomal Theory of Inheritance
Addresses the fact that genes have specific, predicted locations(loci) on chromosomes and that is the whole chromosome that is segregated during meiosis, not the individual genes
- allows for some genes to be linked together in their inheritance patterns.
What is a Wild Type?
the phenotype typically seen by most individuals for a particular character.
Sex in mammals is determined by the combination of sex chromosomes
- human femals have 2 X chromosomes(therefore 23 pairs of homologous chromosomes)
- human males have 1 X and 1 Y chromosomes(therefore 22 pairs of autosomal homologous chromosomes but sex chromosomes are not homologous(not same loci))
Sex Chromosomes(SRY on Y chromosome)
- the SRY gene on the Y chromosome(far fer genes overall compared to X so smaller) codes for proteins that determine male characteristics
* Y chromosome present = chromosomally “male”
Sex Chromosomes(X linked genes, female)
only found on the X chromosome and females will have 2 copies
- Females can be homozygous or heterozygous for X-linked genes - Females give their X linked genes to both daughters and sons
Sex Chromosomes(X linked genes, male)
Males will have 1 copy of X-linked genes
- Males can not be homozygous or heterozygous(can not be carriers) for X linked genes but can be HEMIZYGOUS - males only give their X linked genes to their daughters.
Sex Chromosomes(x linked diseases)
X-linked recessive diseases are more common in males than females.
Sec Chromosomes(Barr body)
During development of female mammalian somatic cells, one of the 2X chromosomes becomes a Barr Body and is deactivated.
- genes on the Barr body are not expressed. - Deactivation is random for each cell so adult somatic cells could be a mosaic if a female was a heterozygote for an X-linked gene.
one functional X chromosome is required so normal human males will not have any Barr Bodies
Unlinked Genes
on different pairs of chromosomes.
They undergo independent assortment and we expect equal probabilities of gametes that will create parental and recombinant phenotypes.
Linked Genes and parental phenotypes
Expect 100% parental phenotypes but not always the case.
- Crossing over between non-sister chromatids during Prophase I of Meiosis I can create recombinant chromosomes
Recombination Frequency(Linked Genes)
the percentage of recombinant offspring out of all offspring.
- recombination frequency of 50% tells us 2 genes are either on completely different chromosomes(thus independent assortment applies) OR so far apart on the same chromosome so that crossing over occurs 100% of the time between the 2 genes
- Any recombination frequency less than 50% implies 2 genes are linked.
Chromosomal Errors
Non-disjunction occurs when homologous chromosomes in anaphase I meiosis I or sister chromatids in mitosis or anaphase II of meiosis II fail to properly separate
- This causes incorrect chromosome numbers in the resulting daughter cells (aneuploidy)
- For meiosis, if aneuploid cells are used for fertilization, it will create zygotes that are potentially monsomic, 2n-1, (have one fewer chromosome than normal with one pair of homologous chromosomes lacking a partner) or trisomic, 2n+1, (having 3 chromosomes for a given pair)
- Polyploidy refers to having extra entire sets of chromosomes
- Aneuploidy in humans often results in miscarriage or severe defects often causing early death
- Monosomy in humans is lethal except for the X chromosome (see below)
Non-disjunction and Down Syndrome
A common human condition caused by non-disjunction is Down Syndrome in which an individual has 3 copies (trisomy, 2n+1) for chromosome #21
- Most frequently caused by older maternal age but all ages of mothers can have a Down baby
Non-disjunction of the sex chromosome
Non-disjunction of the sex chromosomes results in a viable offspring but with various phenotypic effects (except for males missing the X—OY which is nonviable)
- Monosomy for the X chromosome in human females causes Turner’s Syndrome (XO) which causes sterility and other issues
- An extra X in human males is XXY and causes Kleinfelter Syndrome; individuals are male but sterile and have feminized characteristics
Chromosome structure and mistakes in crossing over
Many genes are typically involved in these types of errors often leading to significant phenotypic effects
- Deletion: Missing chromosomal segments
- Duplication: Segments of chromosomes are improperly duplicated
- Inversion: Segments are present but in reverse order
- Translocation: Segments of chromosomes are not found on the correct chromosome but attached to other pairs of homologous chromosomes
Mendel’s Laws of Segregation and Independent Assortment can explain the behavior of chromosomes during meiosis and subsequent inheritance patterns of genes found on those chromosomes.
During the Metaphase I phase of Meiosis I, pairs of non-homologous chromosomes line up independently from other pairs at the metaphase I plate. They will subsequently separate into different daughter cells with no pattern so that in each newly dividing cell going through meiosis, a different outcome of paternal and maternal combinations for every pair of homologues is possible. Genes on different pairs of chromosomes will therefore be assorted independently from each other since they are linked to the entire chromosome moving through meiosis, not as individual genes. The law of segregation also ensures that entire chromosomes are separated with all of their genes so that each eventual daughter cell and gametes will only have one copy of each allele.
Understand how sex linked genes affect the inheritance patterns comparing males and females.
If a gene is located on one of the sex chromomes (X or Y), the inheritance pattern will be different between males and females since human females have 2 copies of the X chromosome (XX) and human males are XY. Most sex linked genes are on the X chromosome (X linked) since the X chromosome is much larger than the Y. In the case of a X linked genetic disease, if a male receives the disease allele, he can not be a carrier and is called hemizygous. He will have the disease and pass on the abnormal allele to all of his daughters and none of his sons. A woman can be a carrier of a X linked recessive disease and will pass it on to both her daughters and sons. It is more likely for males to have X linked diseases since they can not be masked as carriers.
What is X inactivation in female mammals and what effect can this have on inheritance patterns?
In female mammals, one of the two X chromosomes becomes inactive during early embryonic development (becoming a Barr body). It is random which X chromosome becomes the Barr body in each cell, and the Barr body’s genes are not expressed causing females to be a mosaic of two types—some cells having an active maternal copy of X and some having an active paternal copy of X. This becomes noticeable if the female is heterozygous for an X-linked gene since half of her cells will express one version of the allele while the other half will express the alternative allele. Tortoiseshell cats with their unique coloration pattern is a visible example of a this phenomenon.
Describe the effect on inheritance patterns due to linked genes. How does one know genes are “linked”?
Genes that are located on the same chromosome, particularly those that are closely located near each other, will almost always be inherited together since the whole chromosome is what is duplicated and moved apart during meiosis and all the genes will on one chromosome will stay together. In the case of Morgan’s fruit flies, there is a gene that codes for body color (gray vs. black) and a different gene that codes for wing shape (normal or vestigial). These 2 genes are linked since they are found on the same chromosome. Morgan realized this linkage because in mating experiments between dihybrid females (for both characters) that were gray and normal winged and double mutant males that were black and vestigial, the majority of offspring had parental phenotypes, i.e. gray/normal or black/vestigial. If the genes had been on separate genes, the results would have been 25% each of gray/normal:gray/vestigial:black/normal:black/vestigial.
How are recombinants formed and what role does crossing over have in their recombinant frequencies?
In the experiments referred to in question #4, Morgan did discover that while the offspring were not 100% parental types (i.e. gray/normal or black/vestigial). He did get some offspring flies that had new combinations of phenotypes from the parents and were therefore gray/vestigial and black/normal. These new phenotypes are called recombinants and occur because of crossing over. During prophase I of meiosis I, parts of non-sister chromatids of a homologous pair exchange genes. If the fly is a heterozygote for the genes involved (which was the case with the dihybrid females in Morgan’s experiments), new combinations of alleles will occur in the resulting gametes. The crossing over event is random so that it doesn’t always occur in the same place and with the same frequency but the occurrence of any recombinant phenotypes in the offspring suggest it has occurred.
Understand the effect of chromosomal alterations in structure and number on human conditions. What are the 4 types of chromosome damage?
Some genetic diseases are caused by more than just a single gene mutation. Sometimes whole sections of chromosomes are missing or damaged, affecting many genes. Often this damage occurs during mistakes in crossing over. The different types of chromosome damage are:
- ) Deletion: Chromosome segment missing
- ) Duplication: Chromosome segment is repeated
- ) Inversion: Chromosome segment is reversed on the chromosome
- ) Translocation: Chromosome segment is moved from the original chromosome and attached to a non-homologous chromosome
Other chromosomal disorders are caused by an abnormal chromosome number caused by improper division of the chromosomes either in meiosis I or meiosis II. For example, in individuals with Down Syndrome, they have an extra copy of the 21st chromosome causing a trisomy. This extra copy can cause extensive phenotypic differences in these individuals.
