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IB Biology > Genetics - 3.4 Inheritance > Flashcards

Genetics - 3.4 Inheritance Flashcards

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1
Q

Understandings:

A
  • Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed
  • Gametes are haploid so contain only one allele of each gene
  • The two alleles of each gene separate into different haploid daughter nuclei during meiosis
  • Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles
  • Dominant alleles mask the effect of recessive alleles but co-dominant alleles have joint effects
  • Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles
  • Some genetic diseases are sex linked
  • The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes
  • Many genetic diseases have been identified in humans but most are very rare
  • Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer
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2
Q

Inheritance - Mendelian Genetics (GREGOR MENDEL)

A
  • he experimented on pea plants (1822-1884) in Austria (he was a monk) who was crowned the “father of genetics’

1) First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics

2) Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics

3) These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)

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3
Q

Gametes and fusion of gametes

A

Gametes (one set of chromosomes in the sex cells, gametes in animals) are haploid so contain only one allele of each gene. The 2 alleles of each gene separate into different haploid daughter nuclei during meiosis. Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.

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4
Q

Mendel’s Law of Segregation

A

Alleles of a gene become separated from each other during gamete formation

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5
Q

Law of independent Assortment

A

The segregation of alleles for one gene occurs independently to that of any other

(does not hold true for genes located on the same chromosome = LINKED GENES)

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6
Q

Principle of Dominance

A

Recessive alleles will be masked by dominant alleles

(some genes = co-dominace/independant dominance ie not a complete domiance hierarchy)

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7
Q

Discoveries by Mendel:

A

1)
When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants

2)
When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1
E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1

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8
Q

Mendel’s conclusions (5)

A

1) Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)

2) Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles)

3) Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)

4) Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm

5) For each factor, one version is dominant over another and will be completely expressed if present

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9
Q

Haploid gametes:

A

= haploid sex cells developed by meiosis (ie eggs/sperm)

1) during meiosis 1 - homologous chromosomes are separated into different nuclei prior to cell division

2) as homologous chromosomes = carry same genes = segregation of the chromosomes also separates the allele pairs

3) = as gametes only have one copy of each chromosome = can only carry one allele of each gene

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10
Q

Types of zygosity (3)

A
  1. homozygous
  2. Hetrozygous
  3. hemizygous
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11
Q

Homozygous

A

offspring w/ alleles that are the SAME from both the maternal and paternal alleles

eg
homozygous recissive = rr
Homozygous dominant = RR

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12
Q

Hetrezygous

A

offspring w/ alleles that are DIFFERENT (ie maternal and paternal alleles are different)

eg
Rr

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13
Q

Hemizygous

A

only one allele (for each gene located on a SEX chromosome)

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14
Q

Gene

A

= is a length of DNA that controls a specific characteristic by coding for the production of a protein

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15
Q

Allele

A

one specific form of a gene, differing by one or a few bases and occupying the same gene locus as other alleles of the gene

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16
Q

Phenotype

A

the structure, process, behavior or feature of an organism that is determined by its genotype (ie the alleles of the gene that codes for the phenotype) (eg. eye colour)

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17
Q

Genotype

A

= refers to the genetic make-up of an organism

can be: heterozygous (Tt) or homozygous (TT or tt)

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18
Q

Allele combination = MONOHYBRID (6)

A

1) dominance-recessive
2) incomplete dominance
3) co-dominance
4) lethal alleles
5) multiple alleles
6) sex linkage

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19
Q

Allele combination = DIHYBRID (4)

A

1) Dihybrid unlinked
2) Dihybrid linked
3) Epistasis
4) polygenes

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20
Q

Dominace-recessive (monohybrid)

A

dominant allele masks recessive allele in heterozygote

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21
Q

Incomplete dominace

A

both alleles part expresses

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22
Q

Co-dominace

A

Both alleles fully expressed

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23
Q

Lethal alleles

A

one homozygote embryo not developes

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24
Q

multiple alleles

A

more than 2 alleles for gene

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25
Q

sex linkage

A

alleles on unmatched portion of X sex chromosome

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26
Q

dihybrid unlinked

A

two genes on difference chromosome

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27
Q

dihybrid linked

A

two genes on same chromosome

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28
Q

epistasis

A

gene interaction to make new phenotype

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29
Q

polygenes

A

alleles combining to crete variable phenotype

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30
Q

Genetic crosses

A

the “mating” of species across/down generations (= creating new phenotypes)

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31
Q

The Punnett Square

A

REGINALD PUNNETT = devised a method of calculating all possible combinations of gametes and offspring using a grid structure = punnett square

Results = show the EXPECTED RATIO of the allele combinations for the offspring

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32
Q

Punnett square steps (6)

A

Step 1: Designate letters to represent alleles (dominant = capital letter ; recessive = lower case ; co-dominant = superscript)

Step 2: Write down the genotype and phenotype of the prospective parents (this is the P generation)

Step 3: Write down the genotype of the parental gametes (these will be haploid and thus consist of a single allele each)

Step 4: Draw a grid with maternal gametes along the top and paternal gametes along the left (this is a Punnett grid)

Step 5: Complete the Punnett grid to determine potential genotypes and phenotypes of offspring (this is the F1 generation)

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33
Q

Genotype ratios with punnett squares

A

ONLY POSSIBILITIES = may not reflect the actual trend! -> when comparing predicted outcomes to actual data, larger data sets are more likely to yield positive correlation

(the predicted ration is not often the actual ratio as the outcome of each mating is independent of the previous)

34
Q

Dominant/recessive info

A

Traits do not blend, but are either dominant or recessive - there are only 2 phenotypes.

Recessive alleles are masked by dominant ones when alleles are in the heterozygous

35
Q

Linked vs unlinked genes

A

linked = when 2 genes are on the same chromosome

unlinked = when 2 genes are on different chromosomes (these will be independently assorted)

36
Q

Pedigree charts:

A

Charts of the genetic history of a family over several generations

(key for these diagrams = found on pg 4 of booklet + diagram pages)

37
Q

Determining x-linked inheritance =

A

It is not possible to confirm sex linkage from pedigree charts, as autosomal traits could potentially generate the same results
However certain trends can be used to confirm that a trait is not X-linked dominant or recessive

-> X-linked Recessive
-> X-linked Dominant

38
Q

X-linked Dominant

A
  • If a male shows a trait, so too must all daughters as well as his mother
  • An unaffected mother cannot have affected sons (or an affected father)
  • X-linked dominant traits tend to be more common in females (this is not sufficient evidence though)
39
Q

X-linked Recessive

A
  • If a female shows a trait, so too must all sons as well as her father
  • An unaffected mother can have affected sons if she is a carrier (heterozygous)
  • X-linked recessive traits tend to be more common in males (this is not sufficient evidence though)
40
Q

Determining Autosomal Inheritance

A

Dominant and recessive disease conditions may be identified only if certain patterns occur (otherwise it cannot be confirmed)

-> Autosomal Dominant
-> Autosomal Recessive

41
Q

Autosomal Dominant

A
  • If both parents are affected and an offspring is unaffected, the trait must be dominant (parents are both heterozygous)
  • All affected individuals must have at least one affected parent
  • If both parents are unaffected, all offspring must be unaffected (homozygous recessive)
42
Q

Autosomal Recessive

A
  • If both parents are unaffected and an offspring is affected, the trait must be recessive (parents are heterozygous carriers)
  • If both parents show a trait, all offspring must also exhibit the trait (homozygous recessive)
43
Q

Test-cross (or back-cross)

A

T | TT | Tt
——————–
t | tT | tt

test crosses = can be done to determine an organisms genotype (eg TT, tt, Tt), ratio/likelihood of getting these genotypes and therefore phenotype can be predicted by the ratio displayed on the completed test-cross

44
Q

Models of inheritance: Incomplete dominance

A

= where one allele is not completely dominant over the other (= the heterozygote is a mixture of the 2 alleles )

(eg, snapdragon flowers (red, white, pink))

45
Q

Models of inheritance: complete dominance

A

Most traits = follow classical dominant/recessive pattern of inheritance (ie one allele is expressed over another)
-> in heterozygous state = dominant allele will mask recessive allele
-> in homozygous dominant and heterozygous forms will be phenotypically indistinhnishable
-> the recessive allele will only be expressed in the phenotype when in homozygous state

(Capatilised = Dom. allele
Lowercase = recessive allele)

46
Q

genotype

A

-> the gene combination (ie allele combination) for a specific trait = genotype -> will be homozygous or heterozygous for a particular gene

47
Q

phenotype

A

-> the observable characteristics of a specific tait (ie physical expression) = phenotype -> determined by the genotype AND environmental factors

48
Q

Models of inheritance = co-dominance

A

Both alleles are independently and equally expressed in the (phenotype of a) heterozygote individual -> therefore, have an altered phenotype as the alleles are having a joint effect

when representing alleles, use SUBSCRIPTS for the different co=dominant alleles
(RECESSIVE = STILL LOWERCASE)

49
Q

Genetic Diseases

A
  • caused when mutation to a gene(s) abrogates normal cellular function -> leading to the development of a disease phenotype
    • can be caused by dominant recessive or co-dominant alleles
50
Q

Genetic Diseases - dominant alleles

A

= only requires one copy of a faulty allele
- eg Huntington’s disease

51
Q

Genetic Diseases - recessive alleles

A

= needs 2/both copys of a faulty allele
- eg cystic fibrosis

52
Q

Genetic Diseases - co-dominant alleles

A

= only requires one copy of a faulty allele
- eg sickle cell anaemia

53
Q

Genetic diseases and offspring

A

any allele that adversely affects survival and hence the capacity to reproduce - unlikely to be passes onto offspring

54
Q

Genetic diseases - recessive conditions

A

= more common as: faulty allele can be present in carriers without causing dises

55
Q

Genetic diseases - dominant conditions

A

= often = late onset = doesn’t prevent reproduction and the transfer of the faulty allele

56
Q

sex linked genes

A
  • refers to = when a gene-controlling characteristic is located on a sex chromosome (usually x-chromosome related (as very few genes exist on the shorter y-chromosome)
57
Q

Expression of sex linked traits

A

= predominantly associated with a particular gene (as: sex-linked inheritance patterns differ from autosomal patters due to chromosomes not being paired in males (XY))

58
Q

Expression of sex linked traits - Female

A

2x X chromosomes = they can be homozygous or heterozygous (therefore, statistically x-lined dom traits = more common in women)

59
Q

Expression of sex linked traits - Male

A

1x X chromosomes = hemizygous for x-linked traits

60
Q

Following trends hold true for X-linked conditions:

A
  1. Only females can be carries (a heterozygote for a recessive disease condition (males cannot be heterozygous carriers))
  2. males will ALWAYS inherit an x-linked trait from their mother (they inherit a Y-chromosome from their father)
  3. females can not inherit an X-linked recessive condition from an unaffected father (instead, they must receive his dominant trait)
61
Q

gene mutation rates

A

= a change to the base sequence of a gene that can affect the structure and function of the protein it encodes
- can (mutations) be induced (external factors) or spontaneous (via copying errors during DNA replication

62
Q

external factors that induce gene mutations = MUTAGENS

A
  • radiation
  • chemicals
  • biological agents

Agents which increase the rate of genetic mutations = mutagens = can lead to the formation of genetic diseases

63
Q

carcinogens

A

mutagens that lead to the formation of cancer

64
Q

Lethal Genes

A

= gene mutations that result in a gene product which is not only non-functional by affects an organism’s survival

some are fully dominant and therefore lethal in the heterozygote (normally are eliminated rapidly because their expression is fatal)

recessive = only lethal in FULLY recessive (as in heterozygote = masked by the dominant

65
Q

Sickle cell anemia

A

the allele for sickle-cell anemia shows co-dominance with the normal allele

heterozygotes of this = some normal beta chains and some abnormal = do not who any symptoms unless under severe exercise stress or lack of oxygen

Hb^A HB^A = gets malaria but blood normal
Hb^A Hb^S = protests from malaria, only affected dif O2 is low
Hb^S Hb^S = Dies at an early age from anaemis

Recessive lethal gene for the HOMOZYGOTES (RECESSIVE) = die younger (90% die who have it)

66
Q

Multiple Alleles

A

= genes that have more than 2 alleles (genotype will only have 2!!)

Example = Blood type (A and B are co-dominant and they are both dominant to O, which is recessive)

67
Q

Serum agglutinin

A

= an antibody found in the liquid part of the blood ; it causes foreign antigens (IE THOSE OF THE RED BLOOD CELLS OF A PERSON OF A DIFFERENT BLOOD TYPE) to clump together

68
Q

blood groups + percentages

A

A = 40%
B = 10%
AB = 4%
O = 46%

69
Q

blood groups + genotypes and antigens

A

(GENOTYPES = REFER TO PG 17 OF BOOLKET)
(Genotypes = I^A, I^B, i = MORE THAN 2 ALLELES)

A - A antigen
B - B antigen
AB - A AND B antigen
O - no antigen

70
Q

Blood groups + serum agglutinin (antiboy in blood plasma)

A

A - Anti-B antibodies
B - Anti-A antibodies
AB - No antibodies
O - Both Anti-A and Anti-B antibodies

71
Q

Blood groups + what blood types can they RECIEVE

A

A = A, O
B = B, O
AB = A, B, AB, O
O = O

72
Q

Blood groups + can DONATE blood cells to:

A

A = A and AB
B = B and AB
AB = AB
O = A, B, AB, O (universal donor)

73
Q

how many genotypes can be made from a has which has 3 alleles

A

= 6

74
Q

Sex determination

A

The determination of the sex of an organism is controlled by the sex chromosomes provided by each parent

75
Q

females (xx) =

A

homogametic

76
Q

males (xy) =

A

heterogametic

77
Q

why are ‘Y’ important (sex determination)

A

sex determination is based on the presence or absence of the Y chromosomes (otherwise will always be XX = girls)

78
Q

why can the sex determination ratios be wrong =

A

every fertilisation is a unique event and doesn’t interfere with future fertilisation

79
Q

sex-linked gene (SL)

A

women = XX therefore any bad genes on the X = back up
Men = XY therefore any bad genes on the X = NO back up

eg of these genes:
Humans
- colourbindness
-haemophilia

Fruit flies
- red/white eyes

Cat
- orange/black fur colour

80
Q

Y chromosome is derived from the X chromosome, therefore…

A

y chromosome has lost nearly all of the 640 genes it once shared with the X chromosomes

81
Q

sex linage (DEFINITION)

A

= when a gene controlling a characteristic is on the X OR Y chromosome

82
Q

Tortoise shell cats (multicolour) (why can only female cats be it?)

A

Cats must be heterozygous for a black AND orange coat as the gene is on the X chromosome and 2 copies are required = only FEMALES can be tortoise shell

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