Biology 1B - Genetics Flashcards
what is genetics
the study of the inheritance patterns of biological variation within and between species
what does DNA do and what does variation in DNA cause
encodes all of the information necessary to define the function of a single cell, tissue and whole organism
variation in DNA underlines the majority of biological variation (human and disease)
what are applications of genetic technologies
defining disease risk
determining biological relationships
identifying species and individuals
manipulating genomes to produce desirable products and phenotypes
what are the impacts of advancing DNA sequencing technology
reduce the time and cost associated with sequencing complex genomes
revolutionise our ability to understand the role of genetic variation in biology
what can DNA sequencing technology help gain understanding of
human/disease variation, drug response/resistance, cancer predisposition and evolution, nature of microbiome, infectious diseases and species
who first quantified the basic concepts of heredity and how did they do this
Gregor Mendel
using pea plants he described how the observed variation could be understood as a combination of alleles acquired by an individual as the product of the random combination of the single copy of gene from the two parents
what does co dominant mean
where two different alleles are both fully expressed in the phenotype
what does homozygous and heterozygous mean
homozygous = two identical alleles for a specific gene
heterozygous = two different alleles for a specific gene
what does F1 mean
first generation
offspring from cross between two parent organisms
f1 will be heterozygous
what does f2 mean
second generation
cross between two f1 individuals
can be homo or heterozygous
what does one factor cross mean
genetic cross focusing on one gene with two alleles
state Mendel’s first law
law of segregation: each individual has two alleles for a gene, and these alleles segregate (separate) during gamete formation, with each gamete receiving only one allele
how does Mendel’s first law relate to meiosis
during meiosis, homologues chromosomes (carrying alleles for a gene) separate in anaphase, ensuring that each gamete receives only one allele
what is Mendel’s second law
alleles of different genes assort independently of one another during gamete formation, meaning the inheritance of one genes alleles does not affect that of others (as long as the genes are unlinked)
how does Mendel’s second law relate to meiosis
independent assortment occurs during metaphase 1 of meiosis when homologous chromosomes line up randomly at metaphase plate. this leads to different combinations of alleles being distributed into gamates
how does a punnet square for two factor cross work
two factor cross involves two genes e.g.. AaBb x AaBb
4x4 grid with four possible gametes from each parent on each axis
what is the basis of sex determination in the fruit fly, Drosophila melanogaster
sex is determined by the ratio of X chromosomes to sets of autosomes
1:1 ratio (e.g. XX in diploid organism) = female
0.5:1 ratio (e.g. XY or XO) = male
unlike mammals the presence of Y chromosome in Drosophila doesn’t determine maleness (but affects fertility)
what is the basis of sex linkage for genes located on sex chromosomes
sex linkage refers to genes located on sex chromosomes (mainly X)
since males have only one X chromosome the allele on the \x chromosome will be expressed in their phenotype weather dominant or recessive
what is an example of sex linked inheritance
in Drosophila, the white eye mutation is X-linked. Males carry the mutation and display white eyes, females require two mutant alleles to express the trait
describe segregation patterns of alleles from two physically linked genetic loci
linked genes are located close together on the same chromosome and tend to be inherited together because they don’t assort independently
however, crossing over during meiosis can create new combinations of alleles, altering segregation patterns
what is the role of meiotic recombination in altering the segregation patterns of alleles from two physically linked genetic loci:
meiotic recombination: the exchange of genetic material between homologous chromosomes during prophase 1 (crossing over)
this process can break the physical linkage between alleles resulting in new combinations
important for producing heterozygous offspring and natural selection
describe how physical distance between genetic loci affects meiotic recombination
the closer the loci are, the less likely recombination will occur between them, leading to reduced recombinant offspring
important for producing hereozygous offspring
describe the basic chromosomal organisation of the human genome
23 pairs of chromosomes, total of 46 chromosomes
22 autosomes - carry most genetic info
1 pair of sex chromosomes - XX or XY
genome encodes 20,000-25,000 genes
describe the role chromosomes play in sex determination in males
XY, the presence of the SRY gene on the Y chromosome triggers male development by producing the male steroid hormone testosterone initiating formation of testes
describe the role chromosomes play in sex determination in females
XX, absence of SRY gene causes synthesis of oestradiol development of female characteristics
during reproduction, what determines sex of offspring
the sperm, either carrying X or Y chromosome
what are some disorders associated with abnormal numbers of sex chromosomes
Turner syndrome: females single X chromosome
Klinefelter syndrome: males with extra X chromosome (XXY)
describe X linked recessive inheritance
Males XY more frequently affected as they only have one X chromosome
describe X linked dominant inheritance
Females XX may also be affected if one of the defective alleles inherited
describe Y-linked inheritance
traits controlled by genes on Y chromosome passed from fathers to sons only
what is non paternity and its impact on Y chromosome inheritance
non-paternity: when biological father is not as presumed
impact: disrupts Y chromosome inheritance tracing which can lead to incorrect lineage assumptions
so, cannot provide accurate risk assessments on inherited disease
what is the molecular basis of X linked haemophilia
caused by mutations in genes on the X chromosome that encode clotting factors
A - clotting factor VIII
B - clotting factor IX
males (XY) are affected as only 1 X
females can be carriers if one X is unaffected
what are some other sex-linked disorders
x-linked = haemophilia, red-green colour blindness (recessive), ret syndrome (dominant)
y-linked = rare and typically affect male fertility/development
what can segregation errors during meiosis cause
zygotes with different numbers of chromosomes (aneuploidy), causing disease or lethality
what are examples of live births of conditions with incorrect numbers of chromosomes
trisomy 21 (down syndrome)
monosomy X (turner syndrome)
trisomy 18 (Edwards syndrome)
trisomy 13 (Patau syndrome)
(all other ones are lethal)
describe aneuploidies of sex chromosomes
tend to be less severe as Y chromosome is not essential and only contains small numbers of genes
also, variation in X chromosome number is also well tolerated due to the dosage compensation mechanism that already exists to normalise X chromosome gene expression between males and females
what is sickle cell anaemia
- autosomal recessive genetic disease cause by a single base substitution mutation in the beta-haemoglobin gene
what does the mutation in sickle cell amenia do
missense substitution changing a wild type glutamic acid residue (GAA) to valine in the sickle cell allele (GTA)
this amino acid change leads to a complex biochemical and cellular cascade that causes a variety of symptoms
why does sickle dell amenia mutant allele have such high frequency in some parts pf the world
because the sickle cell allele protects against malaria
thus areas where malaria is endemic, heterozygotes have an advantage and balancing selection maintains both alleles in the population
what does aneuploidy mean
abnormal number of chromosomes
what is trisomies and monosomies mean
trisomies - additional chromosome
Monosomies - missing chromosome
describe the basis of down syndrome and its origin in nondisjunction
caused by extra copy of chromosome 21
arises due to nondisjunction mutation where chromosome 21 fails to separate properly during meiosis, leading to egg and sperm with two copies of chromosome 21
resulting zygote has 3 copies
how does sickle cell anaemia provide protection against malaria
heterozygous individuals for the sickle allele are protected because infected red blood cells with some HbS are more likely to be destroyed before the parasite can complete its life cycle
what is a clone
organism produced asexually that is genetically identical to its parent
what is parthenogenesis
form of asexual reproduction where organism develops from an unfertilised egg
what is apomixis
asexual reproduction in plants where seeds are produced without fertilisation
what is a hermaphrodite
An organism that has both male and female reproductive organs, allowing self-fertilization or reproduction with others
what are daphnia
water flies that switch between asexual and sexual reproduction based on different environmental conditions
Explain the roles of asexual and sexual reproduction in Daphnia
asexual: in favourable conditions, to rapidly increase population size and exploit resources. offspring are clones
sexual: harsh conditions, to produce genetically diverse offspring that may better survive in these environments
what are the benefits and costs of asexual reproduction
benefits: rapid reproduction, genetically identical offspring ensuring traits persist
costs: lack of genetic variation reduces adaptability to changing environments
what are the benefits and costs of sexual reproduction
benefits: genetic variation
reduces accumulation of harmful mutations
costs: slower reproduction due to mate finding
energy investment into mating and offspring rearing
Explain the role of sex in the generation of recombinant genotypes
Sexual reproduction generates recombinant genotypes through:
Meiosis - Homologous chromosomes undergo recombination creating new combinations of alleles
fertilisation - The fusion of genetically distinct gametes further enhances genetic diversity.
Discuss evidence that species using only asexual reproduction may have evolved relatively recently from sexual species, using examples:
asexual species have limited genetic diversity compared to sexual
e.g. whiptail lizards (Aspidoscelis), show evidence of recent evolution from sexually reproducing ancestors. Their genomes still carry vestigial signs of sexual processes.
why is evolving from sexual to asexual an evolutionary dead end
- lack of genetic variation in asexual limiting adaptability, making them susceptible to extinction
- accumulation of deleterious mutations (mullers ratchet) leading to decline of fitness
what is Panama disease and what causes it
fungal disease of bananas causes by soil borne fungus fusarium oxysporum
infects plants vascular system
causes wilting and death
risks of genetic identity of a host species
genetically identical populations are equally susceptible to infectious disease, a single pathogen can devastate entire population
advantages of genetically diverse host population
have a range of genetic traits, increasing likelihood that some individuals will resist infection
reduces spread and impact of disease
what is the red queen hypothesis
suggests that species must continuously evolve to survive because their competitors, predators, and pathogens are also evolving
why do pathogens evolve more quickly than their hosts
Pathogens frequently have much shorter life cycles than their hosts and often exist in very large numbers
facilitating evolution of new pathogen variants
give some examples of recently evolved human pathogens
SARS-CoV-2
HIV
Influenza
what does SRY do
SRY encodes a transcription factor that regulates the expression of other genes. The presence of SRY causes the male phenotype; its absence causes the female phenotype.
how was the SRY gene identified
from genetic information derived from people with Klinefelter syndrome and Turner syndrome
what is the indifferent stage
before the SRY gene is expressed there is no physical difference between males and females
internal gonads and external genitals are identical in males and females here
what is the indifferent gonad
the early gonad
what are transcription factors
proteins that regulate gene expression by binding to specific DNA sequences near target genes, they control weather gene is turned on or off
what Major Anatomical Changes Convert the male Indifferent Gonad into Male gonad
initiated by SRY gene
Sertoli cells form gonad and secrete AMH to regress Mullerian ducts (future female structures)
Leydig cells produce testosterone promoting development of wolffian ducts
what Major Anatomical Changes Convert the female Indifferent Gonad into female gonad
occurs in absence of SRY gene
Mullerian ducts persist developing into fallopian tubes, uterus and vagina
indifferent gonad becomes ovaries, while wolffian duct regresses
Anatomical Changes That Convert Indifferent Genitals into Male genitals
genital tubercle elongates to form penis
urogenital folds fuse to form urethra
labioscrotal swellings fuse to form scrotum
Anatomical Changes That Convert Indifferent Genitals into female genitals
The genital tubercle becomes the clitoris.
The urogenital folds remain unfused, forming the labia minora.
The labioscrotal swellings develop into the labia majora.
what does polyspermy mean
fertilisation by more than one sperm
what are the main stages of fertilisation
- sperm binds to receptors on the zona pellucida (outer layer of egg)
- enzymes released from sperm break down this layer
- sperm and egg membranes fuse allowing sperm nucleus to enter cytoplasm
- calcium ions are released in egg triggering development process
how is polyspermy avoided
fast - immediately after fertilisation, egg membrane depolarises preventing more sperm binding
slow - A cortical reaction releases enzymes that alter the zona pellucida to become impermeable to other sperm.
what are the key early stages of development
1) zygote
2) cleavage - rapid mitotic divisions without growth forming blastomeres
3) blastula stage - forms as hollow ball of cells (blastocyst) containing future embryo and extra embryonic tissues
4) gastrulation - cells migrate to form ectoderm, mesoderm and endoderm
what do the 3 germ layers give rise to
ectoderm - skin, nervous system, sensory organs
mesoderm - muscles, bones, blood vessels, heart, reproductive organs
endoderm - digestive tract, respiratory system, liver, pancreas
what is the significance of the trophoblast in mammalian embryos
Trophoblast: The outer layer of the blastocyst that contributes to the formation of the placenta.
it supports implantation into uterine wall and facilitates nutrient and gas exchange between mother and embryo
In non-placental animals, embryos rely on yolk for nutrients.
Structure and Main Functions of the Placenta:
structure: Comprised of both maternal tissue (decidua) and embryonic tissue (from the trophoblast).
Contains chorionic villi for nutrient and waste exchange.
Function: nutrient delivery, waste removal, gas exchange, hormone production, protection
what does cell linage mean
describes both the pattern off cell divisions by which a particular group of cells are born, and the processes of differentiation that make it different to other cell types
what is determination
the process by which a cell commits to a specific type, even before visible changes occur
what is differentiation
The process by which a determined cell develops specialized structures and functions
what is morphogenesis
process that can generate the shape and stricture of tissues/organs/organisms
describe how genes control development
by expressing specific proteins needed for growth and differentiation
describe how genes are regulated
by external signals such as cell communication and environmental factors
This responsiveness ensures that development is tightly coordinated and adapts to internal and external conditions.
what role do model organisms play in our current understanding of molecular principles of development
good models for experiments
e.g.. Drosophila melanogaster
it is genetically simple, short life cycle and conserved genes
what are homeotic genes
Regulatory genes that control the identity and position of body segments or structures during development. They act as “master regulators” by specifying how segments develop into specific anatomical parts.
what are two phenotypes associated with mutation in the key regulators of development.
Antennapedia mutation in Drosophila: Causes legs to develop in place of antennae on the fly’s head
Ultrabithorax mutation in Drosophila: Results in the transformation of the third thoracic segment into a second thoracic segment
two pairs of wings
what are some key principles of development that apply to a wide range of organisms
cell signalling
pattern formation
gene regulation
differentiation
