Genetics：练习题

In DNA, which base always pairs with Guanine (G) through weak hydrogen bonds?

A student is extracting DNA from fresh strawberries. After mashing the fruit and mixing it with a solution containing detergent and salt, the mixture is filtered. What is the next essential step to precipitate the DNA out of the filtrate?

Mendel's work on monohybrid crosses showed that inheritance is determined by discrete units. If a homozygous dominant purple-flowered pea plant (\(PP\)) is crossed with a homozygous recessive white-flowered pea plant (\(pp\)), and the resulting \(F_{1}\) generation is self-pollinated, what percentage of the \(F_{2}\) generation is expected to be heterozygous?

In a species of plant, the allele for red flowers \(R\) is dominant over the allele for white flowers \(r\). A scientist crosses a heterozygous red-flowered plant with a white-flowered plant. What is the probability that an offspring chosen at random will have the genotype \(rr\)?

In human genetics, the sex of an offspring is determined at fertilisation. A Punnett square shows that a male gamete can carry either an \(X\) or a \(Y\) chromosome, while a female gamete always carries an \(X\). What is the ratio of female to male offspring produced over many fertilisations?

Compare the outcomes of mitosis and meiosis in terms of the number of daughter cells produced and the genetic relationship between those cells and the parent cell.

In a genetic cross between two parents heterozygous for a recessive condition (genotype \(Bb\)), calculate the percentage probability that their offspring will be homozygous recessive. Show your reasoning.

In a pedigree analysis for a recessive genetic disorder, two healthy parents have a child with the condition. Explain the genotypes of the parents and the probability that their next child will also be affected, using a Punnett square description.

In a pea plant, the allele for tall stems (\(T\)) is dominant over the allele for short stems (\(t\)). A homozygous tall plant is crossed with a homozygous short plant to produce the \(F_{1}\) generation. Two plants from the \(F_{1}\) generation are then crossed.
(a) State the genotype of the \(F_{1}\) generation.
(b) Use a Punnett square to determine the ratio of tall plants to short plants expected in the \(F_{2}\) generation.

(a) In humans, the allele for cystic fibrosis is recessive (f) and the allele for normal mucus production is dominant (F). Using a Punnett square, determine the probability of two heterozygous parents having a child with cystic fibrosis.
(b) Explain how the sex of the offspring is determined by the inheritance of X and Y chromosomes during fertilisation.