Mendelian Inheritance

Question 1

reasons for working with peas

Answer 1

many varieties of characters (colour) and trait (purple)
short generation time
large number of offspring from each mating
easy to observe and record
could strictly control mating

Question 2

true breeding

Answer 2

over many generations of self pollination plants produce only the same variety as the parent plant e.g. true breeding purple plant will give successive generations of purple flowered plants through self pollination

Question 3

hybridisation

Answer 3

mating or crossing of two true breeding varieties
parents - P generation
F1 - first filial generation - offspring of hybrid mating
F2 - second filial generation - allowing F1 hybrids to self pollinate

Question 4

P generation

Answer 4

parent generation in hybridisation process - mating of two true breeding varieties
YY x yy (hybrid)
F1 are their offspring

Question 5

F1 and F2 generation

Answer 5

F1 - first filial generation / F2 second filial
F1 offspring of 2 true breeding hybridisation
F2 - offspring of 2 F1s
Mendels findings were as a result of him following through to the F2 stage

Question 6

what enabled Mendel to discover the basic patterns of inheritance

Answer 6

If he had stopped with the F1 generation he would not have been able to work out the pattern of dominant and recessive traits as these only become evident with the F2 generation

Question 7

what are Mendel’s two laws of hereditary

Answer 7

the law of segregation

the law of independent assortment

Question 8

what was the main theory about inheritance before Mendel

Answer 8

the blending model which stated that genetic material contributed by two parents mixed and that over successive generations mating will led to a uniform population

Question 9

when did Mendel do is work

Answer 9

Began in 1857

Question 10

the law of segregation

Answer 10

alternative versions of genes account for variations in inherited characters
for each character an organism inherits two copies (two alleles) of a gene from each parent
if the two alleles at the locus differ, then one is the dominant allele (determines the organisms appearance)
the other recessive allele has no noticeable effect on the organisms appearance
the two alleles for a heritable character segregate during gamete formation and end up in different gametes

Question 11

Give an example of the law of segregation for pea flower colour

Answer 11

cross pea flowers with purple and white flowers
F1 - all purple (the purple colour is dominant so all flowers have PP or Pp
F2 - proportion of 3:1 purple flowered to white flowered

The heritable factor for the recessive trait has not been lost but was masked by the presence of the factor for purple flowers

Question 12

alleles

Answer 12

version of a gene e.g. purple flowers or white flowers

at a particular locus on the DNA

Question 13

what is a gene

Answer 13

a sequence of nucleotides at specific locus along a chromosome

Question 14

how can we have different alleles for a particular gene

Answer 14

genes are sequence of nucleotides
there can be variations in the nucleotide sequence
this variation can affect the function of the encoded protein the thus the inherited character of the organism

Question 15

diploid

Answer 15

has two sets of chromosomes- one set inherited from each parent
a genetic locus is represented twice in a diploid cell once on each homologue of a specific pair
of chromosomes

Question 16

somatic cell

Answer 16

all non sex cells

Question 17

describe the arrangement of genes on chromosomes

Answer 17

each diploid cell has two sets of chromosomes - one set inherited from the mother and one from the father
so a gene is represented twice - once on the chromosome inherited from the mother and once on the chromosome inherited from the father
the variety of the gene - the allele - can be the same on both chromosomes (PP)
or they can be different (Pw)

Question 18

what happens if the alleles for a trait differ

Answer 18

the dominant allele determines the appearance of the organism
the other allele is recessive and so not seen

Question 19

homozygote

Answer 19

an organism that has a pair of identical alleles for a gene encoding a character
they are homozygous for that gene

Question 20

homozygous

Answer 20

have two identical alleles for a gene e.g. PP or pp

Question 21

heterozygote / heterozygous

Answer 21

an organism that has two different alleles for a gene
heterozygotes produce gametes with different alleles
e.g. Pp

Question 22

phenotype

Answer 22

organism’s observable traits

this can be different to the genotype because some genes might be recessive and therefore not observable

Question 23

genotype

Answer 23

genetic makeup
can be different from phenotype
e.g. PP and Pp plants have the same phenotype but different genotypes

Question 24

test cross

Answer 24

breeding an organism of unknown genotype with a recessive homozygote to reveal the genotype of the organism

Question 25

monohybrid cross

Answer 25

looking at only one genetic character e.g. colour of flowers

Question 26

dihybrid cross

Answer 26

cross looking at two characters e.g. pea colour and shape

Question 27

how many different combinations are possible from a dihybrid cross

Answer 27

16 (4 x 4) | Ratio 9:3:3:1

Question 28

law of independent assortment

Answer 28

two or more genes sort independently each pair of alleles segregates independently of any other pair of alleles during gamete formation (some genes are inherited together if they are close to each other on the chromosome )

Question 29

multiplication rule

Answer 29

``` to determine the probability of an event - multiply the probability of one event by the probability of the other event i.e. probability of purple flowers (1:2 or half) probability of white (1:2 , half) = half multiplied by half is one quarter so one quarter PP one quarter Pp One quarter Pp one quarter pp ```

Question 30

addition rule

Answer 30

the probability that two or more mutually exclusive events will occur is calculated by adding their individual probabilities e.g quarter of wrinkled peas + 1/4 of yellow =

Question 31

complete dominance

Answer 31

when the alleles of the gene are different the dominant allele observed in all cases the phenotypes of the heterozygote and the dominant homozygote are indistinguishable

Question 32

incomplete dominance

Answer 32

when the alleles are different the phenotype is a blend (somewhere between the two parental varieties) e.g. snapdragons - when red snapdragons are crossed with white some of the offspring are pink (those with one dominant and one recessive)

Question 33

ratios for phenotype and genotype

Answer 33

PP x pp results in 3 purple and 1 white in appearance so the phenotype ratio is 3:1 however the genetic ratio is different results in 1/4 PP 1/2 Pp and 1/4 pp so the ratio is 1:2:1

Question 34

codominance

Answer 34

Codominance refers to a type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual. That is, instead of one trait being dominant over the other, both traits appear, such as in a plant or animal that has more than one pigment colour.

Question 35

codominance example

Answer 35

red flower + white flower = RR red flower rr white flower Rr - red petals and white petals

Question 36

pleiotropy

Answer 36

most genes have multiple phenotypic effects single gene affect a number of characters e.g. cystic fibrosis / sickle cell disease Mendel - same gene that causes flower colour also causes the colour of the seed coating

Question 37

epistasis

Answer 37

one gene affects the phenotype of another because the two genes interact the phenotypic expression of a gene at one locus alters that of another gene at a second locus e.g. labradors - colour gene is affected by the pigment depositing gene /if the dog is recessive at the pigment locus then it is yellow - no pigment regardless of the other gene

Question 38

quantitative characters

Answer 38

characters like human skin colour or height that are graduations along a continuum

Question 39

polygenic inheritance

Answer 39

two or more genes that work together to make a single phenotypic character e.g. height - 180 genes impact on height skin pigmentation in humans is controlled by many separately inherited genes

Question 40

pedigrees

Answer 40

collecting information about a families history for a particular trait / creating a family tree use for calculating the probability of future children inheriting a trait / having a particular genotype or phenotype

Question 41

carriers

Answer 41

heterozygotes (two forms of the gene) though phenotypically normal can transmit the recessive allele to their offspring most people who have recessive disorders are born to parents who are carriers of the disorder but have normal phenotype

Question 42

examples of diseases caused by recessive genes | passed on through carriers

Answer 42

``` cystic fibrosis (4% are carriers) sickle cell disease ```

Question 43

examples of diseases caused by dominant alleles

Answer 43

achondroplasia (dwarfism) people who do not have it are homozygous for the recessive allele Most lethal dominant gene diseases are not passed on because the organism dies before reproducing some like Huntington's disease only appear after reproductive age

Question 44

when does the segregation of alleles occur in meiosis

Answer 44

anaphase 1 - when alleles separate

Question 45

who found proof in chromosomes that supported Mendel's theory

Answer 45

Morgan (early 1900s) working with fruit fly - Drosophilia

Question 46

why was the choice of Drosophila good

Answer 46

prolific breeders - many offspring and mate often only 4 chromosomes chromosomes can be seen with a light microscope three autosomes and one pair of sex chromosomes

Question 47

wild type

Answer 47

phenotype for a character most commonly observed | in natural populations

Question 48

mutant phenotypes

Answer 48

alternatives to wild type

Question 49

homologous chromosomes

Answer 49

two chromosomes in a pair carry genes controlling the same inherited characters carry the same genes

Question 50

somatic cells

Answer 50

all the cells in the body except gametes

Question 51

sex chromosomes

Answer 51

chromosomes involved in sex determination | all other chromosomes are called autosomes

Question 52

diploid cell

Answer 52

any cell with two chromosome sets has a diploid number of chromosomes -2n humans - diploid number is 46 drosophila - 8 (2 x 4)

Question 53

haploid cell

Answer 53

cells tat have a single set of chromosomes gametes are haploid cells for humans the haploid number is 23 drosophila - 4

Question 54

X - Y system for sex determination

Answer 54

``` Females have XX Males XY (Y smaller) ``` each egg receives one x each sperm can have x or Y Fathers pass X to all their daughters but none to their sons

Question 55

sex linked gene

Answer 55

a gene located on either sex chromosome chromosomes on the X chromosome are called X linked (1100 in humans) chromosomes in the Y chromosome are Y linked (78 in humans) - mainly related to function of testicular function X chromosome genes for many characteristics

Question 56

hemizygous

Answer 56

because females have XX they can be homozygous or heterozygous for a particular allele on a X chromosome because males have only on X chromosome they only have one form of the allele - they are hemizygous (hemi - they only have half)

Question 57

why do more males than females have X linked disorders

Answer 57

Females have two X chromosomes so can be XX Xx Xx and xx if x is recessive have 1/4 chance of inheriting If a male has only one X it can be X or x - so have 1/2 chance if the male receives a recessive six linked gene from their mother they will express the trait

Question 58

examples of sex linked traits

Answer 58

colour blindness muscular dystrophy haemophilia baldness

Question 59

Barr body

Answer 59

females have XX so twice as many of the genes located on the X chromosome as males in females one X is inactivated during embryonic development As a result males and females have the same effective dose (protein making ability) of most X linked genes the inactive X forms the Barr body

Question 60

why don't females - XX- produce double the proteins for genes on the X chromosome as males (XY)

Answer 60

inactivation of one X chromosome during embryonic development As a result both males and females have the same dosage of most X linked genes.

Question 61

linked genes

Answer 61

genes located near each other on the same chromosome tend to be inherited together in genetic crosses genes that are genetically linked (different from Mendel's law of independent assortment

Question 62

parental types

Answer 62

offspring that have a phenotype that matches either of the phenotypes of the parents e.g. round yellow pea X green wrinkly pea = round yellow and green wrinkly offspring which are parental types but also yellow wrinkly and green round which are recombinants this is for genes that are not linked

Question 63

recombinants

Answer 63

offspring that have new combinations of phenotypes e.g. round yellow pea X green wrinkly pea = round yellow and green wrinkly offspring which are parental types but also yellow wrinkly and green round which are recombinants

Question 64

nondisjunction

Answer 64

when members in a pair of homologous chromosomes do not move apart properly during meiosis 1 or sister chromatids fail to separate during meiosis II. One gamete receives two of the same type of chromosome and another gamete receives no copy

Question 65

aneuploidy

Answer 65

a zygote with an abnormal number of chromosomes monosomic - missing one chromosome trisomic - having three of a chromosome monosomy and trisomy are estimated to occur in 10-25% of human conceptions and are the main reason for pregnancy loss

Question 66

monosomic

Answer 66

aneuploid with a missing chromosome

Question 67

trisomic

Answer 67

aneuploid with an extra chromosome

Question 68

down syndrome

Answer 68

trisomy21

Question 69

polyploidy

Answer 69

``` have more than two complete chromosome sets triploidy - 3n tetraploidy - 4n Common in the plant kingdom some fish and amphibians are polyploid ```

Question 70

monohybrid ratios

Answer 70

P- true breeding HH X hh F1 - all express dominant allele 1 : 1 : 1 = Hh Hh Hh Hh F2 - 3 : 1 HH Hh Hh hh