inheritance Flashcards

Question

why are males more susceptible to sex-linked disorders?

Answer 1

- males only have **one sex-linked gene locus** (bc only have 1 X chromosome) -> they are neither homozygous nor heterozygous for sex-linked loci, but instead is **hemizygous** - as males possess only a **single X chromosome, whatever allele present on the X chromosome of males will be directly expressed in the phenotype**, whether or not the allele is dominant or recessive. - so X linked disorders mainly affect males as they are hemizygous for recessive X-linked disorders, - **males only need one copy** of the defective recessive allele to have the disorder, whereas **females require 2 copies** of the defective recessive allele

Answer 2

- haemophilia - red-green colour blindness - duchenne muscular dystrophy

Answer 3

- an **affected father** will **transmit** the recessive X-linked allele to **all daughters**, whom become carriers (i.e. hetereozygous for the trait), but not to any sons, because his son will inherit his Y chromosome only and the X chromosome from his mother - if a **carrier female** has a child with a **normal male**, there is a **50% chance each daughter will be carrier** and a **50% chance each son will have the disorder** - if a **carrier female** has a child with an **affected male**, there is a **50% chance that each child, regardless of sex, will have the disorder**. daughters who don't display the recessive phenotype will be carriers, whereas males without the disorder will be completely free of the recessive allele

Answer 4

a reciprocal cross is a **pair of crosses** in which the **traits of the 2 parents are reversed**. eg cross 1: red eye female, white eye male. cross 2: white eye female, red eye male a reciprocal cross can be conducted to discern if a trait is carried on a sex chromosome or on an autosomal chromsome

Answer 5

if the trait is X-linked, the reciprocal crosses will yield **non-identical results** if the trait is autosomal, the reciprocal crosses will yield **identical results**

Answer 6

- if an offspring displays a trait although both parents lack the trait, it means the trait is **recessive** - if both parents are affected and all children are affected, the trait is **recessive** - if traits skip generations, the trait is **recessive** - if all offspring of unaffected parents are unaffected, the trait is **dominant** - if traits do not skip generations, the trait is **dominant** - if most affected individual are male, the trait is **X-linked** - if all daughters of an affected father are affected, the trait is **X-linked and dominant** (assuming mother is normal)

Answer 7

linked genes are genes that control different characters and are **situated on the same chromosome at different loci** ## Footnote linked genes will not produce normal dihybrid inheritance ratios of 9:3:3:1 (double heterozygote cross) or 1:1:1:1 (test cross)

Answer 8

- the **2 pairs of genes** are located **very close together on the same chromosome** and are **closely linked** together - **no crossing over** occurs between the 2 gene loci so they are **transmitted together as a unit** into the same gamete, resulting in only **parental gametes** - the 2 phenotypes formed are both **recombinant phenotypes**

Answer 9

- the genes are **located some distance apart on the same chromosome**, and thus can be separated when **crossing over** occurs during meiosis - during prophase I of meiosis, crossing over may occur between the 2 gene loci, so there will be genetic recombination leading to **a new combination of alleles in gametes** - the separated chromatids eventually end up in separate gametes, and after fertilisation, give rise to new combinations of alleles in the offspirng, which are called **recombinants** - since crossing over is a random process, **majority** of offspring produced will show **parental allele combinations** and hence **parental phenotypes** and a **minority of recombinant allele combinations**, and hence **recombinant phenotypes** ## Footnote the probability of crossing over occurring between 2 gene loci increases with increasing distance between the 2 gene loci

Answer 10

conduct a test cross between a **double heterozygous individual with a double homozygous recessive individual** if the genes are present on **different chromosomes**, - offspring of 4 different phenotypes should be produced with a ratio 1:1:1:1 if the genes are **completely linked**, - only 2 phenotypes are produced - offspring should consist of the 2 parental phenotypes with a ratio 1:1, with no recombinant phenotypes if the genes are **incompletely linked** - 4 different phenotypes are produced, 2 parental and 2 recombinant - there will be a larger percentage of parental phenotypes & smaller percentage of recombinant phenotypes

Answer 11

coupling is when 2 dominant alleles are on 1 chromosome, and the 2 recessive alleles are on the homologous partner. AB/ab repulsion is when the dominant allele is linked with a recessive on one chromsome. Ab/aB

Answer 12

COV & RF are also known as the **percentage of crossing over.** COV/RF = no. of recombinants/total no. of offsprin x100 - the chance of crossing over occurring betwen **2 linked genes** is proportional to the **distance between them** - if the 2 linked genes are **far apart** on a chromosome, there is a **greater statistical chance** that crossing over will **separate** them than if they were closer -> **greater proportion of recombinants** - so the distance between genes can be determined by the **proportion of recombinants** ## Footnote less recombinants = genes are closer

Answer 13

**phenotypic variance** (Vₚ) of a population has two components, **genotypic variance** (VG) and **environmental variance** (VE) VE is most easily determined when studying homozygous organisms, as VG = 0 VG is due to genotype, and includes the effects of additive genes, dominant genes & epistasis

Answer 14

gene mutation - an inheritable **change in nucleotide sequence of DNA**, which occurs at a **single gene locus** on a chromosome, resulting in the formation of **new alleles** - gene mutation can occur by deletion (removing 1 or more nucleotides), insertion (adding 1 or more nucleotides) or substitution (replacing one nucleotide with another) chromosomal mutation - a change in the **structure** of a chromosome OR - a change in the **number** of chromosomes meiosis/sexual reproduction - **crossing over of non-sister chromatids** of homologous chromosomes during **prophase I** of meiosis, resulting in **new combinations** of paternal & maternal alleles in each chromatid - **independent assortment of homologous chromosomes** during **metaphase I**, resulting in **random distribution** of maternal & paternal chromosomes in each gamete sexual reproduction - the fusion of 2 haploid gametes to form a diploid zygote restoring the diploid number is **random**, resulting in genetic variation. there is also random mating between individuals in a population ## Footnote genetic variation from meiosis & sexual reproduction results in new COMBINATIONS of existing alleles only MUTATIONS result in NEW ALLELES

Answer 15

- himalayan rabbits with white body, black ears, nose, feet and tail are **homozygous for the cʰ allele** of the tyrosinase gene - the tyrosinase gene codes for a **heat-sensitive form of the enzyme tyrosinase**, which is needed for melanin production, resulting in black fur - heat sensitive tyrosinase is **active only when temperature is below 33C** - only in parts of the body that are cool enough, eg in **extremities** (nose, ears, feet, tail) does black fur grow ## Footnote experiment: himalayan rabbit with white body, black ears, nose, feet & ears had the fur on the back shaved, and an ice pack fixed on the shaved region. when the ice pack is left in position for weeks and kept cold, black fur, instead of the original white fur, begins to grow below the ice pack

Answer 16

- bee colonies have 3 types of bees: queen, drones & workers - drones are males and develop from **unfertilized haploid eggs** - the queen & workers are females and develop from **fertilised diploid eggs** - although the queen & workers have the **same amount of genetic material, they are phenotypically different** - this difference is due to the **diet of be larvae**. after hatcching, all the larvae are initially fed with **royal jelly**. larvae destined to be worker bees are switched to a diet containing **honey and pollen**, while larvae destined to be the queen are fed with **royal jelly** - the high protein content in royal jelly stimulates the formation & maturation of the **female reproductive system**

Answer 17

**2 independently assorting genes** may interact to influence a **single character** ## Footnote same ratio as dihybrid (9:3:3:1), but dihybrid is when 2 characteristics are controlled by 2 genes but gene interaction is when 1 characteristic is controlled by 2 or more genes

Answer 18

epistasis is inferred when the expression of an allele of one gene **suppresses/inhibits** the expression of alleles of a different gene (the hypostatic gene). ## Footnote when 2 or more genes influence one characteristic, an allele at one gene locus may have an overriding effect on the genotype at a **different locus**

Answer 19

for recessive epistasis, **2 recessive alleles** at the epistatic gene locus will suppress/inhibit the effect of either allele of the hypostatic gene at a different locus ## Footnote phenotypic ratio of **9:3:4** with a double heterozygous cross 9 (A-B-) : 3 (A-bb) : 4 (3 aaB- + 1 aabb) aa suppresses alleles B/b

Answer 20

for dominant epistasis, **one dominant allele** at the epistatic gene locus will suppress/inhibit the effect of both alleles of the hypostatic gene at a different locus ## Footnote phenotypic ratio of **12:3:1** with a double heterozygous cross 12 (9 A-B- + 3 AAb-) : 3 (aaB-) : 1 (aabb)

Answer 21

for duplicate recessive epistasis, **2 recessive alleles** at either of the 2 gene loci will suppress/inhibit the effect of the dominant allele at the other locus ## Footnote phenotypic ratio of **9:7** with a double heterozygous cross 9 (A-B-) : 7 (3 aaB- + 3 A-bb + 1 aabb)

Answer 22

variation describes the recognisable **differences in characteristics** between organisms of the same natural population or species

Answer 23

***discontinuous variation*** phenotypes are **definite and clear cut**, and can be divided into distinct groups, with **discrete phenotypic classes** observed. **intermediates are not observed** ***continuous variation*** phenotypes are not clear cut and cannot be divided into distinct contrasting group, with a **range of phenotypes** observed. **intermediates are observed**

Answer 24

***discontinuous variation*** variation is controlled by a **single or a few gene(s)**. this gene may have 2 or more alleles ***continuous variation*** variation is controlled by the combined effect of **multiple genes**, known as **polygenic inheritance** gene act on the phenotype in an **additive manner**. effects of single genes are too slight to be detected. the **combined effect** of these genes can produce individuals of infinite phenotypic varieties

Answer 25

***discontinuous variation*** there is **little or no environmental effect** on the phenotypic expression of the gene(s) ***continuous variation*** phenotypes can be modified by the **cumulative effect of varying environmental factors acting on the different genotype** although the genotype of an organism is determined at the time of fertilisation, the **degree of expression** allowed to this genetic potential hinges on **environmental factors** during the development of the organism

Answer 26

***discontinuous variation*** phenotypic measurements are normally represented on **bar graphs** measurement is **qualitative**, analysed by making **counts & ratios** ***continuous variation*** phenotypic measuremens form a **normal distribution curve** that has a continuous range of intermediate forms between 2 extremes. these are normally represented on **histograms** measurement is **quantitative**; statistical analyses give estimates of **population parameters** such as the mean and standard deviation

Answer 27

DNA replication is asexually reproducing organisms is so highly accurate and almost error-free, that there is little possibility of variation in the genotype. any **apparent variation** between asexually reproducing organisms is almost always the result of **environmental influences** ## Footnote does NOT generate new alleles, only new COMBINATIONS of alleles

Answer 28

1. **crossing over** between non-sister chromatids of homologous chromosomes during prophase I of meiois 2. **independent assortment** of bivalents at the metaphase plate during metaphase I of meiosis 3. **random fertilisation** producing a variety of combinations of alleles ## Footnote does NOT generate new alleles, only new COMBINATIONS of alleles new alleles are generated by **mutations** (for both asexual & sexual), which refer to a change in the structure of a gene or in the structure/number of chromosomes. only mutations that occur during the **formation of gametes** can be inherited, producing distinct differences between individuals and fors the basis of **discontinuous variation**

Answer 29

the chi-squared test is a statistical test for the **significance of data** that consists of **discontinuous/discrete** variables

Answer 30

1 - state the hypotheses - H₀: there is **no significant difference** between the observed and expected ratios. (it assumes that **any differences are due to chance**) - Hᴀ: there is **significant difference** between the observed and expected ratios. (it assumes that **differences are not due to chance**) 2 - calculate the expected number of individuals for each phenotypic class, using the expected phenotypic ratios 3 - calculate the chi-squared value, using the sum of the square of the difference between expected & observed over the expected numbers (formula will usually be given) 4 - compare the calculated chi-squared value against a chi-squared probability table. take the **level of significance to be 0.05** unless stated otherwise. **degree of freedom (df) = n-1**, where n is the number of phenotypic classes. 5 - conclude - if **x²calc > x²crit**, the probability that **chance alone** is the reason for the difference between observed & expected results is **less than 5%**. the deviation is **significant**, so reject H₀ in favour of Hᴀ - if **x²calc < x²crit**, the probability that **chance alone** is the reason for the difference between observed & expected results is **more than 5%.** the deviation is **not significant**, so do not reject H₀.