The chromosomal basis of inheritance
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| Mendelian inheritance has its physical basis in the ___ of chromosomes. | behavior
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| ___ genes exhibit unique patterns of inheritance. | Sex-linked
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| Alterations in chromosome ___ or ___ cause some genetic disorders. | - number
- structure
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| genes | segments of DNA located along chromosomes
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| the chromosome theory of inheritance | - a basic principle in biology stating that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns
- AS CHROMOSOMES GO, SO DO THE GENES
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| The arrangement of chromosomes at metaphase I of meiosis and their movement during anaphase I account for the ___ and ___ of the alleles for different characters. | - the segregation
- independent assortment
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| The first evidence associating a specific ___ with a specific ___ came from the work of Thomas H. Morgan. | - gene
- chromosome
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| Why did Morgan choose the fruitfly as his experimental organism? | - a single mating produces many offspring
- new generations can be bred often
- they have only 4 pairs of chromosomes, which can be easily seen with a light microscope
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| After some two years of breeding work, what did Morgan discover amongst the fruit flies he was examining? | a mutation (suddenly, a male fruit fly had white eyes instead of red)
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| wild type | the phenotype for a character most commonly observed in natural populations (such as red eyes in fruit flies)
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| mutant phenotype | - traits that are alternatives to the wild type (such as white eyes in fruit flies)
- they are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele
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| Explain where the designations w+ and w came from. | - the letter is based off of the mutant phenotype (w = white eyes)
- the + identifies the allele for the wild type trait (w+ = red eyes)
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| What happened when Morgan crossed a white-eyed male with a wild-type (red-eyed) female? | - the F1 generation all had red eyes
- so, red eyes are the dominant trait
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| What happened when Morgan crossed an F1 red-eyed female with a F1 red-eyed male? | - the F2 generation shown a typical Mendelian ratio of 3 red-eyed flies to 1 white-eyed fly
- however, no females displayed the white-eye trait; all white-eyed flies were males
- mutant trait sex-linked to X chromosome!
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| A chromosome in a diploid organism is hemizygous when ___. | - only one copy is present
- (for example, male fruit flies are hemizygous for the eye color gene; it's only on the X chromosome)
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| How did Morgan's work provide support for the chromosome theory of inheritance? | - his finding of the correlation between a particular trait and an individual’s sex / a specific gene is carried on a specific chrom. (eye-color gene on the X-chrom.)
- also indicated that genes located on a sex chrom. exhibit unique inheritance patterns
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| What did Morgan's work show about inheritance patterns when genes are located on a sex chromosome? | - the phenotypes are not equally distributed on male and female offspring
- fathers pass X-linked alleles to all of their daughters but to none of their sons
- mothers can pass X-linked alleles to both sons and daughters
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| Describe the chromosomal basis of sex in humans. | - there are 2 kinds of sex chromosomes: X and Y
- the Y-chromosome is much smaller than the X
- the sex of the organism depends on whether the sperm cell contains an X- or Y-chromosome
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| Describe the chromosomal basis of sex in grasshoppers, cockroaches, and some other insects. | - only one type of sex chromosome: the X
- females are XX, males are X
- sex determined by whether the sperm cell contains an X-chromosome or nothing
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| Describe the chromosomal basis of sex in birds, some fishes and some insects? | - the sex chromosomes present on the egg determine the sex of the offspring
- females are ZW, males are ZZ
- (the opposite of humans)
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| Describe the chromosomal basis of sex in most bees and ants? | - there are no sex chromosomes
- females develop from fertilized eggs (diploid)
- males develop from unfertilized eggs (haploid); have no father
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| What is on the Y chromosome that has to do with the development of testes? | - the Y-chromosome has a gene on it that is required for the development of testes: SRY (sex-determining region of Y)
- in the absence of SRY, the gonads develop into ovaries
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| sex-linked genes | genes located on either sex chromosome
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| linked genes | genes located close enough together on a chromosome that they tend to be inherited together
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| What is so different about the inheritance of Y-linked and X-linked genes? | - X-linked genes can be present in both males and females
- Y-linked genes can only be present in males
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| If a color-blind woman married a man who had normal color vision, what would be the probable phenotypes of their children? | - so, X^n X^n x X^N Y
- 50% chance to have a child with color blindness, 50% chance to have a carrier
- all male children will be colorblind, all females carriers
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| What is Duchenne muscular dystrophy and how does it work? | - a human genetic disease characterized by progressive weakening and a loss of muscle tissue
- caused by a sex-linked recessive allele
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| What is hemophilia and how does it work? | - a human genetic disease characterized by excessive bleeding following injury
- caused by a sex-linked recessive allele resulting in the absence of one or more blood-clotting proteins
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| The cells of female and male mammals have the same effective dose (one copy) of most X-linked genes. How does this happen if females have two X chromosomes and males have only one X chromosome? | - most of one X-chromosome in each cell in female mammals becomes inactivated during early embryonic development
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| Barr body | - a dense object lying along the inside of the nuclear envelope in cells of female mammals, representing a highly condensed, inactivated X-chromosome
- become reactivated during gamete formation
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| When does X inactivation in female mammals occur and what does that mean? | - one X inactivated during early embryonic development
- its random which X-chrom. will be inactivated in the embryonic cells present at that time
- so, females consist of a mosaic of 2 types of cells (active X from father or active X from mother)
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| What’s the deal with X inactivation in tortoiseshell cats? | - you can clearly see the resulting mosaic from X inactivation
- female cats will have random patches of black and orange fur
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| What is nondisjunction and how does it result in abnormal chromosome numbers? | - an error in meiosis in which the members of a pair of homologs do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II
- so, some gametes might have one less or one more chromosome than they should
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| What happens in meiosis I nondisjunction? What is the result? | - homologous chromosomes fail to separate during anaphase I
- the end result is 2 gametes with an extra chromosome (n+1) and 2 with one less (n-1)
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| What happens in meiosis II nondisjunction? What is the result? | - sister chromatids fail to separate during anaphase II
- the end result is 2 normal gametes (n), 1 with an extra chromosome (n+1), and 1 with one less (n-1)
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| aneuploidy | a chromosomal aberration (deviation from the norm) in which one or more chromosomes are present in extra copies OR are deficient in a number
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| What would you get if a n+1 sex cell united with an n sex cell? | - aneuploidy (one extra chromosome)
- zygote would be trisomic (2n+1)
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| What would you get if a n-1 sex cell united with an n sex cell? | - aneuploidy (one less chromosome)
- zygote would be monosomic (2n-1)
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| What is polyploidy? How does it happen? | - a chromosomal alteration in which the organism possesses more than two complete chromosome sets
- it is the result of an accident of cell division
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| How do you get a triploid? | - chromosomal sets of 3...3n
- may arise by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes
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| How do you get a tetraploid? | - chrom. sets of 4...4n
- could result from the failure of a 2n zygote to divide after replicating its chrom.s; subsequent normal mitotic divisions would then produce a 4n embryo
- fertile; has pairs of chrom.s, so meiosis can occur (allodiploids cant)
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| What is an alloploid? | - a hybrid individual or cell with two or more sets of chromosomes derived from two different ancestral species
- can be allodiploid, allotriploid, etc.
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| What is Down syndrome? | - aneuploid condition (2n+1); 47 chromosomes instead of 46
- extra #21 chromosome (trisomy 21)
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| What is Klinefelter syndrome? | - aneuploid condition (2n+1); 47 chromosomes instead of 46
- extra X-chromosome in a male (XXY)
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| What is Jacob's syndrome? | - aneuploid condition (2n+1); 47 chromosomes instead of 46
- extra Y-chromosome (XYY)
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| What is trisomy X? | - aneuploid condition (2n+1); 47 chromosomes instead of 46
- extra X-chromosome in a female (XXX)
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| What is Turner syndrome? | - aneuploid condition (2n-1); 45 chromosomes instead of 46
- only one X-chromosome
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| What is cri du chat? | - disorder caused by altered chromosome
- deletion in chromosome #5
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| What is chronic myelogenous leukemia? | - disorder caused by altered chromosome
- reciprocal translocation between chromosome #9 and #22
- shortened chromosome #22 is then called the Philadelphia chromosome
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