Table of Contents
10TH CLASS MOST IMPORTANT OBJECTIVE QUESTIONS ON GENETICS
WHAT DO YOU MEAN BY GENETICS
ICSE CLASS 10TH MOST IMPORTANT MCQ ON GENETICS . The scientific study of heredity. Genetics pertains to humans and all other organisms. So, for example, there is human genetics, mouse genetics, fruit fly genetics, etc
I. The basic rules of inheritance were first demonstrated by Mendel
A. at the time of Mendel’s work, most thought that parental traits were fluids that “blend” in offspring
B. Mendel recognized that this model did not explain what he observed
C. Mendel chose a model system and carefully established testing conditions
- he used pea plants that he could outcross or allow to self-fertilize
- he chose traits that had two clear possible outcomes (yellow or green seeds, etc.)
- he established true-breeding or “pure” lines to use for genetic crosses
D. terminology for genetic crosses - P generation (or P1) = parental generation
- F1 generation = first generation offspring (from filial)
- F2 generation = second generation offspring
- phenotype – appearance or characteristic of an organism
- genotype – genetic makeup of an organism, determines phenotype
- gene – unit of heredity; controls a trait that determines a phenotype
- locus – the location of a particular gene on a chromosome
- alleles – alternative versions of a gene
- dominant – allele that dominates over others in determining phenotype
- recessive – allele whose phenotypic expression is “hidden” when a dominant allele is present
- hybrid – offspring from a cross between two “pure” lines of different, competing phenotypes
II. rules and terminology for examination of genetic inheritance
A. Mendel’s law of segregation - when Mendel crossed pure lines of different, competing phenotypes, he found that the F1 generation was uniform and matched one of the parents’ phenotypes
● example: P1 yellow seed X green seed all F1 yellow seed - when F1 plants were crossed or selfed, the F2 plants had both P1 phenotypes in a ratio of roughly 3:1
● using offspring from above F1 X F1 F2 3 yellow seed: 1 green seed - thus, contrary to the popular belief of the time, recessive traits are not lost in a mixing of parental phenotypes – they are merely hidden in some “carrier” individuals
- Mendel explained these ratios with what we now call his law of segregation; stated in modern terms: individuals normally carry two alleles for each gene, these alleles must segregate in production of sex cells
- later investigations of cell division revealed the mechanism for segregation: the pairing and subsequent separation of homologous chromosomes during meiosis
B. genotype vs. phenotype - phenotype is the actual appearance or characteristic, and is determined by genotype; knowing the phenotype will not always directly reveal the genotype (recessive traits can be masked)
- genotype is the listing of the actual alleles present; if you know the genotype, you should be able to predict the phenotype
▪ genotypes are either homozygous or heterozygous
▪ homozygous – the homologous chromosomes have the same allele at the locus in question
▪ heterozygous – the homologous chromosomes have different alleles at the locus; if there is a dominant allele the trait of the dominant allele will be expressed
● the same letter is used to indicate all alleles (superscripts or subscripts are sometimes needed, if there are more than 2 alleles known)
● DOMINANT ALLELES ARE CAPITALIZED; recessive alleles are lowercase
C. Punnett square – way of diagramming genetic crosses that uses the laws of probability
D. more terminology - test cross – mating an individual that has the dominant phenotype for a trait with an individual with the recessive phenotype; this often will reveal the genotype of the dominant parent, or at least give some idea of the probably genotype
- monohybrid cross – cross between individuals that are both heterozygous for the gene that you are following; note that these give a 3:1 phenotype ratio and a 1:2:1 genotype ratio
E. law of independent assortment - dihybrid cross – cross between individuals that are both heterozygous for two different genes that you are following
- when Mendel performed dihybrid crosses he found phenotype ratios of 9:3:3:1, which is explained by the product rule
- this led to Mendel’s law of independent assortment: segregation of any one pair of alleles is independent of the segregation of other pairs of alleles
- crossing over breaks linkages between genes
● recall crossing over during prophase I between homologous chromosomes; it is the only way to get genetic recombination between genes that are on the same chromosome
● the further apart two genes are, the more likely they are to have crossing over occur between them (thus leading to genetic recombination)
III. Sex determination and sex chromosomes
A. sex determination varies between species - hermaphroditic organisms – have both sexes in the same individual
- many animals have sex determined in response to environmental signals
- most animals have sex determined by genetic inheritance; sex chromosomes are involved
B. sex chromosomes - homogametic sex – has a pair of similar sex chromosomes; all gametes that individual produces get that kind of sex chromosome
- heterogametic sex – has two different sex chromosomes, and makes gametes with two different types of sex chromosome
- all the other, non-sex chromosomes are called autosomes
- usually, the sex chromosome found in the homogametic sex is considerably larger, and the shorter sex chromosome found only in the heterogametic sex has few genes
- in humans, females are XX and males are XY (not all do it this way – birds are essentially reversed in this)
- X and Y chromosomes have regions of homology (sequence similarity) that allow for pairing during meiosis I
- usually, but not always, the sex determining gene is on the Y chromosome
● XXY humans are male (Klinefelter syndrome)
● X_ humans are female (Turner syndrome)
C. sex-linked traits - genes on sex chromosomes show inheritance patterns that do not fit traditional Mendelian ratios that describe what happens to genes on autosomes
- in humans (and other species with XY sex determination), a gene found only on the X chromosome is said to be X- linked (which is a type of sex-linked)
● males only get one X chromosome, from the mother, and are hemizygous at every locus found only on the X chromosome
● thus, recessive X-linked alleles are expressed more often in males than in females
● X-linked alleles are written with superscripts
● we will work examples for X-linked inheritance in class; be sure to try some on your own
IV. Using genetics in breeding
A. inbreeding – the mating of closely related individuals (includes self-fertilization) - typically done to enhance a desirable trait (quantitative or qualitative) that an individual has
- also done to produce homozygous lines (“true-breeding”)
- often produces genetically inferior individuals due to unmasking of deleterious recessive traits
B. outbreeding – mating of essentially unrelated individuals (unclear cut-off, beyond second cousins is generally considered enough) - hybrid vigor – progeny produced by outbreeding often show a clear genetic superiority as a group over their parents when the parents are from mostly inbred lines
- the exact cause of hybrid vigor is not clear, and likely has multiple aspects
● less expression of deleterious recessive traits certainly plays a role
● heterozygote advantage – some positive attribute that is not found in any homozygous case
▪ for example, sometimes expression of one of the allelic forms is good to have under one condition and the other is good under a different condition; expressing both allelic forms allows the organism to do well in both conditions, which may both come up during its life
C. family pedigree analysis - pedigree – a chart summarizing phenotypes and/or genotypes within a family over several generations
- standard symbols for pedigrees:
● generations are designated with capital roman numerals, starting with the oldest generation at the top
● each generation gets one row, and genetic parents are connected by a horizontal line
● males are square, females round
● each individual gets a number, going from left to right for each generation
● a vertical line connects parents to their offspring
● coloring is used to indicate phenotype (and, sometimes, known genotypes) - pedigree analyses only work well when a single locus is involved in determining a phenotype (so-called Mendelian traits); still, many disorder genes have been identified and characterized with the help of pedigree analysis (some human genetic disorders will be discussed later in this unit)
- you need to be able to analyze pedigrees and determine which is the most likely mode of inheritance for a single-gene trait among these choices: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive
D. karyotyping - many genetic problems occur on the large-scale, chromosomal level
- studies of karyotypes are often done to test for such problems
- a karyotype display reveals the composition of chromosomes for an individual
● a cell sample is taken (white blood cells, amniocentesis, chorionic villus sampling, etc.)
● cells are grown in culture, and eventually treated to make chromosomes easy to photograph
● the chromosome images are then analyzed and used to create the karyotype display - chromosomes are identified by size, position of the centromeres, and staining patterns
E. human genome project - sequencing the human genome provides a means to greatly accelerate studies of human genetics
● the underlying genetic causes for gene-based traits can be studied more easily (including traits that involve multiple genes)
● sequence variations can be readily analyzed
● more sophisticated genetic testing can be performed, leading to the potential for genetically tailored medical treatment - a “complete” draft of the human genome sequence (~3 billion basepairs) was made public in April 2003 [coinciding with the 50th anniversary of the Watson and Crick paper announcing the structure of DNA] – there are ~25,000 genes in the genome, based on current interpretations of the sequence
sickle cell anemia
● most common in those of African descent; about 1 in 500 of African-Americans have it
● caused by a mutation in hemoglobin that makes it tend to crystallize when oxygen is not bound to it
▪ makes red blood cells take on a sickle shape, which can slow or even block blood flow through veins and capillaries
▪ can damage tissues due to lack of oxygen and nutrients, and is very painful
▪ shortens lifespan of red blood cells, leading to anemia (low red blood cell count)
● treatments have increased life expectancy, including stimulating fetal hemoglobin production and bone marrow transplants; work continues on gene therapy
ICSE CLASS 10TH MOST IMPORTANT MCQ ON GENETICS
- Which of the following is totally impossible outcome of Mendel’s Experiment?
a. 3 tall 1 short plant
b. 24 tall and 8 short plants
c. 8 tall and 0 short plants
d. 4 tall plants and 1 medium height plant. - Which of the following is not a direct conclusion that can be drawn from Mendel’s Experiment?
a. Only one parental trait is expressed
b. Two copies of each trait is inherited in sexually reproducing organism
c. For recessive trait to be expressed, both copies should be identical
d. Natural selection can alter frequency of an inherited trait. - Which one is a possible progeny in F2 generation of pure bred tall plant with round seed and short plant with wrinkled seeds?
a. Tall plant with round seeds
b. Tall plant with wrinkled seeds
c. Short plant with round seed
d. All of the above - Which section of DNA provides information for one protein
a. Nucleus
b. Chromosomes
c. Trait
d. Gene - Which of the following is not controlled by genes?
1. Weight of a person
2. Height of a person
a. only 1
b. only 2
c. both 1 and 2
d. sometimes 1 and sometimes 2 - What is the probability that the male progeny will be a boy?
a. 50%
b. 56%
c. 47.43%
d. It varies - Who have a perfect pair of sex chromosomes?
a. Girls only
b. Boys only
c. Both girls and boys
d. It depends on many other factors - With whom you can associate theory of evolution?
a. Charles Darwin
b. Mendel
c. Stanley miller
d. Harold Urey - Which of the following can be called a characteristic?
a. Plants can photosynthesis
b. We have 2 eyes
c. Mango tree is multicellular
d. All of these - A cross between a tall pea-plant (TT) and a short pea-plant (tt) resulted in progenies that were all tall plants because
(a) tallness is the recessive trait.
(b) shortness is the dominant trait.
(c) height of pea-plant is not governed by gene T or t.
(d) tallness is the dominant trait. - The number of pairs of sex chromosomes in the zygote of a human being is
(a) 2
(b) 3
(c) 1
(d) 4 - A zygote which has an X-chromosome inherited from the father will develop into a
(a) girl
(b) boy
(c) either boy or girl
(d) X-chromosome does not influence the sex of a child. - A man with blood group A marries a woman having blood group O. What will be the blood group of the child?
(a) O only
(b) A only
(c) AB
(d) Equal chance of acquiring blood group A or blood group O. - What does the progeny of a tall plant with round seeds and a short plant with wrinkled seeds look like?
(a) All are tall with round seeds.
(b) All are short with round seeds.
(c) All are tall with wrinkled seeds.
(d) All are short with wrinkled seeds. - If a round, green seeded pea-plant (RRyy) is crossed with a wrinkled yellow seeded pea- plant (rrYY), the seeds produced in F1 generation are
(a) round and green
(b) round and yellow
(c) wrinkled and green
(d) wrinkled and yellow - Which of the following decides the sex of the child?
(a) male gamete, i.e., sperm
(b) female gamete, i.e., ovum
(c) both sperm and ovum
(d) mother - Pure-bred pea plant A is crossed with pure bred pea plant B. It is found that the plants which look like A do not appear in F1 generation but re-emerge in F2 generation. Which of the plants A and B are tall and dwarf?
(a) A are tall and B are dwarf.
(b) A are tall and B are also tall.
(c) A are dwarf and B are also dwarf
(d) A are dwarf and B are tall - In humans if gene B gives brown eyes and gene b gives blue eyes, what will be the colour of eyes of the persons having combinations
(i) Bb and (ii) BB
(a) (i) Blue and (ii) Brown
(b) (i) Brown and (ii) Blue
(c) (i) Brown and (ii) Brown
(d) (i) Blue and (ii) Blue - A cross between two individuals results in a ratio of 9 : 3 : 3 :1 for four possible phenotypes of progeny. This is an example of a
(a) Monohybrid cross
(b) Dihybrid cross
(c) Test cross
(d) F1 generation - Which of the following characters can be acquired but not inherited?
(a) Colour of skin
(b) Size of body
(c) Colour of eyes
(d) Texture of hair
ANSWERS
- d
- d
- d
- d
- b
- a
- a
- a
- d
- d
- a
- a
- d
- a
- b
- a
- c
- c
- b
20.b
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