B5.1 Flashcards
What is a gamete?
An organism’s reproductive cell (egg in females, sperm in males), which has half the number of chromosomes (23).
What is a chromosome?
A structure found in the nucleus made up of a long strand of DNA.
What is a gene?
A short section of DNA that codes for a protein and contributes to a characteristic.
How are characteristics controlled by genes?
Some characteristics are controlled by a single gene (e.g. fur colour in mice,
but most characteristics result from many genes interacting
What is an allele (or variant)?
Different forms of the same gene. Humans inherit one allele from each parent, so they have two alleles for each gene.
What is a dominant allele?
Only one copy is needed for it to be expressed and for the phenotype to be observed.
What is a recessive allele?
Two copies are needed for it to be expressed and the phenotype to be observed.
What does homozygous mean?
When both inherited alleles are the same (e.g. two dominant or two recessive alleles).
What does heterozygous mean?
When one allele is dominant and the other is recessive.
What is genotype?
The combination of alleles an individual has, e.g. Aa.
What is phenotype?
The physical characteristics observed in the individual, e.g. eye colour.
What is the genome?
All the genetic information of an organism.
How do the genome and environment affect traits?
They influence how genetic traits are presented — this results in phenotypes.
What is continuous variation?
Variation such as height — controlled by multiple genes and affected by the environment, a range of categories
What is discontinuous variation?
Variation such as eye colour — distinct categories, often controlled by single genes
How has studying the genome helped science?
It has improved understanding of genetic diseases, treatment of inherited disorders, and tracing human migration.
What causes genetic variance?
All variants (alleles) are caused by mutations.
Do all mutations affect phenotype?
No — most mutations occur in non-coding DNA and don’t affect proteins.
When can a mutation affect phenotype?
If it’s in a coding region, it may change protein structure and activity, potentially changing phenotype.
What happens if a mutation is in coding DNA?
It can alter protein structure, e.g. changing enzyme active sites so substrates can’t fit.
What happens if a mutation is in non-coding DNA?
It may affect gene expression by stopping transcription of mRNA.
What is an advantage of sexual reproduction?
Produces variation in offspring.
Why is variation helpful?
Increases chance of survival if environment changes — gives a survival advantage.
How does variation help species survive?
It reduces the chance of the entire species becoming extinct.
What is another benefit of sexual reproduction?
Allows for selective breeding.
What are advantages of asexual reproduction?
Only one parent needed
Uses less energy
Faster
No need to find a mate
In favourable conditions, many identical offspring can be produced
How does sexual reproduction allow desirable traits?
Two organisms with desirable traits can be bred for stronger offspring.
How does this link to natural selection?
Speeds up natural selection by passing on helpful traits quickly.
What’s an example of selective breeding?
Breeding two animals with lots of meat to increase food production.
What is meiosis?
The formation of 4 non-identical gametes from one cell.
Where does meiosis happen?
In reproductive organs.
What happens first in meiosis?
Cell copies its chromosomes (now diploid).
What happens after chromosomes copy in meiosis?
Cell divides once — each new cell has 46 chromosomes (half from original).
What happens last in meiosis?
Each cell divides again — 4 cells formed with 23 chromosomes each (haploid).
What are the 4 cells called from meiosis?
Gametes.
Why are gametes genetically different?
Because chromosomes are shuffled and sorted randomly
What is a Punnett square diagram used for?
To show probability of offspring inheriting genotypes and phenotypes.
What do capital letters show in Punnett squares?
Dominant characteristics.
What do lowercase letters show in Punnett squares?
Recessive characteristics.
How many chromosome pairs do humans have?
23 pairs.
What does the 23rd pair do?
Carries sex-determining genes.
What are the sex chromosomes for females?
XX — only pass on X chromosomes in eggs.
What are the sex chromosomes for males?
XY — can pass on either X or Y in sperm.
What’s the chance of a baby being male or female?
50%
Who was Gregor Mendel?
A monk who studied inheritance in pea plants.
What did Mendel discover?
Traits are inherited as ‘hereditary units’
One unit from each parent
Units can be dominant or recessive and can’t be mixed
Traits are inherited as ‘hereditary units’
One unit from each parent
Units can be dominant or recessive and can’t be mixed
Genes and chromosomes hadn’t been discovered yet.
How was Mendel’s work understood later?
19th century: Chromosomes seen during cell division
20th century: Genes understood to be on chromosomes
1953: Structure of DNA discovered, explaining gene function
