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Biology 30 > Mendelian genetics > Flashcards

Mendelian genetics Flashcards

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1
Q

Describe the history of genetic modification:

A
  • People have recognized for years that traits are inherited
  • Used this to their advantage in “selective breeding”
  • Purposely mated individual animals (and plants) to select for certain traits
  • Eg. Dog breeds
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2
Q

Who is Thomas Morgan?

A
  • 1933 - discovered the role of genetics on chromosomes in heredity
  • Worked with fruit flies
  • When crossing flies with red eyes he found some offspring has white eyes
  • This caused him to investigate heredity further (up until this point he was doubtful of theories from Darwin and Mendel)
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3
Q

Describe Mendel’s experiments:

A
  • Mendel wanted to test patterns in genetics by observing traits such as flower color
  • He let pea plants self-pollinate for several generations to ensure they were pure bred
  • Then artificially crossed one type of plant with another to observe results – created hybrids (not purebred)
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4
Q

Define monohybrid cross:

A

A cross involving only one trait

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5
Q

What are Mendel’s generations?

A

P - purebred parents generation
F1 - First generation of hybrids
F2 - second generation of hybrids

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6
Q

What happened with the F1 generation?

A
  • Only one trait (of the two possible parent traits) was observed in the offspring
  • Mendel decided that the trait observed must be dominant
  • The trait not observed was recessive
  • In this case one trait showed complete dominance over the other (the recessive trait will not show at all if the dominant gene is present)
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7
Q

What happened with the F2 generation?

A
  • Recessive traits reappeared in some offspring
  • About 1 in every 4 offspring showed recessive traits
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8
Q

Describe the law of segregation:

A
  • “All individuals have two copies of each factor. These copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every factor.”
  • Note: we now know Mendel’s “factors” were genes
  • Different forms of each gene are now called alleles
  • Not all genetic combinations are this simple
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9
Q

Define gene:

A

A genetic code that designates a specific trait eg. Flower color

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10
Q

Define allele:

A

Form of a gene eg. Purple vs. white flowers

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11
Q

Define genotype:

A

Combination of alleles for a trait

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12
Q

Define pheotype:

A

Physical trait observed (characteristic of an organism)

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13
Q

Define homozygous:

A

Two identical alleles

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14
Q

Define heterozygous:

A

Two different alleles (one is likely recessive, but not always)

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15
Q

Define dominant allele:

A

An allele that has the same effect on an organism whether present in homozygous or heterozygous state

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16
Q

Define recessive allele:

A

Only has an effect when present in homozygous state

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17
Q

Define carrier:

A

Has one copy of recessive allele that can cause genetic disease

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18
Q

Describe the relationship between gametes and alleles:

A
  • Gametes are haploid and therefore only have one copy of gene (only one allele)
  • When gametes fuse, diploid zygotes end up with two copies of a gene – may be the same allele or different alleles of that gene
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19
Q

How are genotypes symbolized?

A
  • First letter for description of dominant allele is used
  • Capital letter represents dominant allele
  • Lower case letter (same letter) represents recessive allele
20
Q

Describe punnett squares:

A
  • Used to analyze/predict genetic crosses
  • Used with F1 generation
21
Q

Describe a test cross:

A
  • Sometimes geneticists will do a test cross to determine the genotype of an individual
  • Cross an individual with a dominant phenotype with an individual known to be homozygous recessive
  • Used to check to see if dominant phenotype is a result of homo or heterozygous genotype
22
Q

What is a dihybrid cross?

A
  • Examination of two traits simultaneously
  • Four genes (so two genotypes) are observed, instead of two
23
Q

Describe Mendel’s law of independent assortment:

A
  • The two alleles for one gene segregate (assort) independently of the alleles for other genes during gamete formation
  • Translation: one gene does not generally affect the expression of another
24
Q

How do you analyze a dihybrid cross?

A
  • A punnett square can still be used to predict offspring but sixteen squares rather than four are necessary
  • All possible combinations of the two different genes for females are written across the top, those for the male are written down the side
25
Q

Describe co-dominant alleles:

A
  • Sometimes both alleles can be considered dominant and both can be expressed
  • End up with a mixture of traits
  • Eg. Roan Animal: both black (B) and white (W) are expressed. (Individual black and white hairs). Genotype is BW
  • Capital letters
26
Q

Describe incomplete dominance alleles:

A
  • Neither allele conceals the other
  • End up with a mixed trait eg. Red and white combine to make pink
  • Can be represented with subscript numbers or superscript letters:
    • Eg. Red flowers: R1R1, Pink : R1R2, White: R2R2
    • Eg. Sickle cell anemia, normal: HbAHbA, Some traits of sickle cell: HbAHbS, Full sickle: HbsHbs
27
Q

Describe what happens when there are multiple alleles:

A
  • Some genes have more than two alleles
  • Each individual has only two alleles, but more than two exist within a population
  • Many more patterns are possible than with just two alleles
28
Q

Describe blood types in terms of genetics:

A
  • There are three alleles for blood types (note, also separate gene for Rh factor)
  • Type A and B are dominant (co-dominant) and O is recessive
  • Alleles are IA, IB, and i which form A, B and O respectively, see chart on next slide
  • Rh factor is a separate gene where Rh+ is dominant over Rh-
29
Q

Describe sex-linked alleles:

A
  • Some genes can be linked to one of the sex chromosomes (encoded for by the sex chromosomes)
  • Most common are x-linked traits
30
Q

Describe the inheritance of x-linked traits:

A
  • If an x-linked trait is recessive, the phenotype is more likely to appear in males – why?
  • The gene is not usually present at all on a Y chromosome so there is no chance of the dominant allele there
  • Therefore a male only needs one recessive allele for the trait to appear
31
Q

Describe color blindness:

A
  • An x-linked recessive trait
  • Much more common in males than female
  • Possible genotypes and phenotypes for a female are:
    • XNXN – Normal
    • XNXn – Normal (but a carrier)
    • XnXn – colorblind
  • Possible genotypes and phenotypes for a male are:
    • XNY – normal
    • XnY – colorblind
  • As you can see, there is nothing on the Y chromosome to “dominate” the recessive Xn trait if it does appear
32
Q

How do we represent sex-linked traits?

A
  • Females will be represented by 2 X chromosomes with superscripts to indicate the allele on that chromosome
  • Males will be XY with the appropriate superscripts for the alleles
  • Y does not usually have an allele attached to it, unless it is a y-linked trait
33
Q

Describe Y-linked traits:

A
  • Not as common as females cannot be carriers
  • Because females don’t have Y chromosomes, men can only pass Y-linked genes to their sons
34
Q

Describe barr bodies:

A
  • Structure formed when the inactive X chromosome condenses tightly
  • In females one X-chromosome inactivates and condenses into a Barr body in every cell
35
Q

What happens in the genetics of calico cats?

A
  • Gene for coat color in cats is located on the X-chromosome
  • Calico cats occur when different X-chromosomes in different cells are de-activated into Barr bodies
  • Eg. Cells that produce orange fur have the X chromosome containing “black” de-activated and vice versa
36
Q

Describe lethal alleles:

A
  • What if you get a “non-mendelian” ratio of offspring, such as 2:1
  • Could be caused by a lethal allele
  • Definition: a gene where an allele causes a lethal trait
  • If the organism dies as an embryo, could get unexpected ratios of traits in offspring
  • Death sometimes takes years
  • Can be expressed as incomplete dominance when heterozygous
  • Eg. Manx cats
37
Q

Define chromosome mapping:

A

Process used to determine position of genes on chromosomes

38
Q

Define map unit:

A

Distance between points where crossover is likely to occur

39
Q

Define map distance:

A

Distance between genes on a single chromosome

40
Q

How can we tell if a gene crossover has occured?

A
  • Some genes are linked – encoded for on the same chromosome
  • Linked genes do not assort independently, they assort together
  • Therefore, linked genes in gametes should occur in the same phenotypic combinations as the parents
41
Q

Describe an example of gene crossover:

A
  • Two genes for sunflowers are encoded on the same chromosome – the gene for height and flower color.
  • T = dominant, tall; t= recessive, short; Y = dominant, yellow; y= recessive, red
  • You know that the Tall and Yellow genes are linked (because there is no record of tall red plants or short yellow plants in the pedigree)
  • You cross a tall yellow plant with a short red plant and 5% of the offspring are tall and red or short and yellow
  • The small number of unexpected results indicates crossover must have occurred
42
Q

Describe recombinants:

A
  • Offspring that have different combinations of genes on a chromosome than the parents
  • Recombination frequency – percentage of times crossover occurred when P1 found gametes
  • Recombination frequency = number of recombinant types x 100% divided by Total # offspring
43
Q

How do you use the recombination frequency?

A
  • 1% = one map unit
  • Can examine several genes and compare the distances between the genes to determine the order they come in
  • Our sample calculation allows us to speculate that the genes for color and height of a sunflower are 10 map units apart
  • We examine a third gene, seed type, and find out that it is 3 map units from color and 7 from height
44
Q

What is a pedigree?

A
  • A flow chart used to symbolize ancestry
  • Shows patterns of relationships and traits in a family over many generations
  • A way to study genetics without actually performing artificial crosses
45
Q

Describe the numbers used on a pedigree:

A
  • Roman numerals are used to indicate the generation
  • Arabic (regular) numerals are used to indicate the specific individuals in a generation
46
Q

How do we use a pedigree to study patterns?

A
  • Can use a pedigree to work backwards to follow a certain trait
  • For example, if a child shows up with an autosomal recessive trait, you know both parents must be carriers and at least one of each of their parents must be a carrier

Biology 30 (7 decks)

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